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Document and Entity Information	3 Months Ended
Document and Entity Information	Mar. 31, 2019
Document And Entity Information [Abstract]
Entity Registrant Name	Silk Road Medical Inc
Entity Central Index Key	0001397702
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Document Type	S-1
Amendment Flag	false

Consolidated Balance Sheets - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 15,509,000	$ 24,990,000	$ 33,331,000
Accounts receivable, net	4,096,000	4,520,000	5,215,000
Inventories	8,056,000	5,744,000	3,248,000
Prepaid expenses and other current assets	1,391,000	1,408,000	279,000
Total current assets	29,052,000	36,662,000	42,073,000
Property and equipment, net	2,846,000	2,880,000	486,000
Restricted cash	310,000	310,000	510,000
Other non-current assets	6,460,000	1,029,000	17,000
Total assets	38,668,000	40,881,000	43,086,000
Current liabilities:
Accounts payable	3,134,000	1,252,000	1,546,000
Accrued liabilities	7,658,000	7,586,000	3,109,000
Total current liabilities	10,792,000	8,838,000	4,655,000
Long-term debt	44,597,000	44,201,000	27,589,000
Redeemable convertible preferred stock warrant liability	31,803,000	16,091,000	4,185,000
Other liabilities	4,285,000	1,069,000	0
Total liabilities	91,477,000	70,199,000	36,429,000
Commitments and contingencies (Note 7)
Redeemable convertible preferred stock issuable in series, $0.001 par value
Liquidation preference: $121,144 at December 31, 2017 and 2018	105,265,000	105,235,000	105,235,000
Stockholders' deficit:
Shares issued and outstanding: 1,386,615 and 1,135,310 at March 31, 2019 and December 31, 2018, respectively	1,000	1,000	1,000
Additional paid-in capital	5,194,000	4,557,000	2,977,000
Accumulated deficit	(163,269,000)	(139,111,000)	(101,556,000)
Total stockholders’ deficit	(158,074,000)	(134,553,000)	(98,578,000)
Total liabilities and stockholders’ deficit	38,668,000	$ 40,881,000	$ 43,086,000
Pro forma
Current liabilities:
Redeemable convertible preferred stock warrant liability	0
Redeemable convertible preferred stock issuable in series, $0.001 par value
Liquidation preference: $121,144 at December 31, 2017 and 2018	0
Stockholders' deficit:
Shares issued and outstanding: 1,386,615 and 1,135,310 at March 31, 2019 and December 31, 2018, respectively	25,000
Additional paid-in capital	144,023,000
Accumulated deficit	(163,269,000)
Total stockholders’ deficit	$ (19,221,000)

Consolidated Balance Sheets (Parenthetical) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowances for accounts receivables		$ 1,885,000	$ 611,000
Temporary equity, par value (in USD per share)	$ 0.001	$ 0.001	$ 0.001
Temporary equity, shares authorized (in shares)	24,069,615	24,069,615	24,069,615
Temporary equity, shares issued (in shares)	21,238,105	21,233,190	21,233,190
Temporary equity, shares outstanding (in shares)	21,238,105	21,233,190	21,233,190
Liquidation preference	$ 121,174,000	$ 121,144,000	$ 121,144,000
Common stock, par value (in USD per share)	$ 0.001	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	29,879,220	29,879,220	29,879,220
Common stock, shares issued (in shares)	1,386,615	1,135,310	663,270
Shares outstanding (in shares)	1,386,615	1,135,310	663,270
Pro forma
Temporary equity, shares authorized (in shares)	0
Temporary equity, shares issued (in shares)	0
Temporary equity, shares outstanding (in shares)	0
Liquidation preference	$ 0
Common stock, shares authorized (in shares)	100,000,000
Common stock, shares issued (in shares)	24,571,140
Shares outstanding (in shares)	24,571,140

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenue	$ 12,766	$ 5,706	$ 34,557	$ 14,258
Cost of goods sold	3,339	1,934	10,874	5,129
Gross profit	9,427	3,772	23,683	9,129
Operating expenses:
Research and development	2,707	2,100	10,258	7,242
Selling, general and administrative	13,866	6,319	34,820	20,261
Total operating expenses	16,573	8,419	45,078	27,503
Loss from operations	(7,146)	(4,647)	(21,395)	(18,374)
Interest income	52	13	189	34
Interest expense	(1,352)	(989)	(4,361)	(3,943)
Other income (expense), net	(15,712)	215	(12,063)	2,927
Nett loss	(24,158)	(5,408)	(37,630)	(19,356)
Net loss attributable to non-controlling interest			1	0
Nett loss attributable to Silk Road Medical, Inc. common stockholders	(24,158)	(5,408)	(37,629)	(19,356)
Comprehensive loss	(24,158)	(5,408)	(37,630)	(19,356)
Comprehensive loss attributable to non-controlling interest			1	0
Comprehensive loss attributable to Silk Road Medical, Inc. common stockholders	$ (24,158)	$ (5,408)	$ (37,629)	$ (19,356)
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted (in USD per share)	$ (20.12)	$ (7.31)	$ (39.16)	$ (44.58)
Weighted average common shares used to compute net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted (in shares)	1,200,719	739,308	960,882	434,158
Pro forma net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted (unaudited) (in USD per share)	$ (0.35)		$ (1.07)
Pro forma weighted average common shares used to compute net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted (unaudited) (in shares)	24,380,984		24,139,683

Condensed Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Deficit - USD ($)	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Non-controlling Interest
Temporary equity, beginning balance at Dec. 31, 2016	$ 63,417,000
Temporary equity, beginning balance (in shares) at Dec. 31, 2016	14,350,216
Temporary Equity [Abstract]
Exercise of Series C preferred stock warrants	$ 41,818,000
Exercise of Series C preferred stock warrants (in shares)	6,882,974
Temporary equity, ending balance at Dec. 31, 2017	$ 105,235,000
Temporary equity, ending balance (in shares) at Dec. 31, 2017	21,233,190
Beginning balance at Dec. 31, 2016	$ (80,095,000)	$ 1,000	$ 2,104,000	$ (82,200,000)
Beginning balance (in shares) at Dec. 31, 2016		372,632
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options	$ 338,000		338,000
Exercise of stock options (in shares)	438,578	290,638
Employee stock-based compensation	$ 442,000		442,000
Nonemployee stock-based compensation	93,000		93,000
Net loss	(19,356,000)			(19,356,000)
Comprehensive loss	(19,356,000)			(19,356,000)
Ending balance at Dec. 31, 2017	(98,578,000)	$ 1,000	2,977,000	(101,556,000)
Ending balance (in shares) at Dec. 31, 2017		663,270
Temporary equity, ending balance at Mar. 31, 2018	$ 105,235,000
Temporary equity, ending balance (in shares) at Mar. 31, 2018	21,233,190
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options	$ 284,000		284,000
Exercise of stock options (in shares)		186,944
Employee stock-based compensation	164,000		164,000
Nonemployee stock-based compensation	(2,000)		(2,000)
Net loss	(5,408,000)			(5,408,000)
Comprehensive loss	(5,408,000)			(5,408,000)
Ending balance at Mar. 31, 2018	(103,453,000)	$ 1,000	3,436,000	(106,890,000)
Ending balance (in shares) at Mar. 31, 2018		850,214
Temporary equity, beginning balance at Dec. 31, 2017	$ 105,235,000
Temporary equity, beginning balance (in shares) at Dec. 31, 2017	21,233,190
Temporary equity, ending balance at Dec. 31, 2018	$ 105,235,000
Temporary equity, ending balance (in shares) at Dec. 31, 2018	21,233,190
Beginning balance at Dec. 31, 2017	$ (98,578,000)	$ 1,000	2,977,000	(101,556,000)
Beginning balance (in shares) at Dec. 31, 2017		663,270
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options	656,000		656,000
Exercise of stock options (in shares)		438,578
Employee stock-based compensation	728,000		728,000
Nonemployee stock-based compensation	183,000		183,000
NeuroCo common stock issuance	1,000				$ 1,000
Issuance of common stock in connection with NeuroCo merger (in shares)		33,462
Issuance of common stock in connection with NeuroCo merger	0
Net loss	(37,630,000)			(37,629,000)	(1,000)
Comprehensive loss	(37,630,000)			(37,629,000)	$ (1,000)
Ending balance at Dec. 31, 2018	(134,553,000)	$ 1,000	4,557,000	(139,111,000)
Ending balance (in shares) at Dec. 31, 2018		1,135,310
Temporary Equity [Abstract]
Exercise of Series C preferred stock warrants	$ 30,000
Exercise of Series C preferred stock warrants (in shares)	4,915
Temporary equity, ending balance at Mar. 31, 2019	$ 105,265,000
Temporary equity, ending balance (in shares) at Mar. 31, 2019	21,238,105
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of stock options	$ 375,000		375,000
Exercise of stock options (in shares)	251,305	251,305
Employee stock-based compensation	$ 241,000		241,000
Nonemployee stock-based compensation	21,000		21,000
Net loss	(24,158,000)			(24,158,000)
Comprehensive loss	(24,158,000)			(24,158,000)
Ending balance at Mar. 31, 2019	$ (158,074,000)	$ 1,000	$ 5,194,000	$ (163,269,000)
Ending balance (in shares) at Mar. 31, 2019		1,386,615

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Statement of Cash Flows [Abstract]
Net loss	$ (24,158)	$ (5,408)	$ (37,630)	$ (19,356)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	174	57	517	129
Stock-based compensation expense	262	162	911	535
Change in fair value of redeemable convertible preferred stock warrant liability	15,712	(216)	11,906	(2,958)
Amortization of debt discount and debt issuance costs	11	23	68	89
Amortization of right-of-use asset	162
Non-cash interest expense	471	386	1,555	1,705
Loss on disposal of property and equipment			159	0
Provision for accounts receivable allowances	313	278	1,835	423
Provision for excess and obsolete inventories	27	0	23	63
Changes in assets and liabilities
Accounts receivable	110	2,012	(1,003)	(4,793)
Inventories	(2,339)	(433)	(2,565)	(2,408)
Prepaid expenses and other current assets	16	(387)	(1,128)	(10)
Other assets	(651)	17	(62)	0
Accounts payable	2,287	(244)	(309)	678
Accrued liabilities	52	(897)	3,622	651
Other liabilities	(1,069)	0	406	0
Net cash used in operating activities	(8,620)	(4,650)	(21,695)	(25,252)
Cash flows from investing activities
Purchase of property and equipment	(117)	(455)	(2,276)	(443)
Proceeds from sale of property and equipment			6	0
Net cash used in investing activities	(117)	(455)	(2,270)	(443)
Cash flows from financing activities
Proceeds from long-term debt			15,000	5,000
Proceeds from issuance of common stock	375	284	656	338
Proceeds from exercise of preferred stock warrants	30	0
Payments of deferred offering costs	(1,149)	0	(233)	0
Non-controlling interest			1	0
Proceeds from issuance of redeemable convertible preferred stock, net of issuance costs			0	41,818
Net cash provided by financing activities	(744)	284	15,424	47,156
Net change in cash, cash equivalents and restricted cash	(9,481)	(4,821)	(8,541)	21,461
Cash, cash equivalents and restricted cash, beginning of year	25,300	33,841	33,841	12,380
Cash, cash equivalents and restricted cash, end of year	15,819	29,020	25,300	33,841
Supplemental disclosure of cash flow information
Cash paid for interest	870	580	2,738	2,149
Non-cash investing and financing activities:
Accounts payable and accrued liabilities for purchases of property and equipment	22	90	6	14
Landlord paid tenant improvements			794	0
Unpaid deferred offering costs	1,204	$ 0	$ 717	$ 0
Right-of-use asset obtained in exchange for lease obligation	$ 3,982

Formation and Business of the Company	3 Months Ended	12 Months Ended
Formation and Business of the Company	Mar. 31, 2019	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Formation and Business of the Company	Formation and Business of the Company The Company Silk Road Medical, Inc. (the “Company”) was incorporated in the state of Delaware on March 21, 2007. The Company has developed a technologically advanced, minimally-invasive solution for patients with carotid artery disease who are at risk for stroke. The Company’s portfolio of TCAR products enable a new procedure, referred to as transcarotid artery revascularization, or TCAR, that combines the benefits of endovascular techniques and surgical principles. The Company manufactures and sells in the United States its portfolio of TCAR products which are designed to provide direct access to the carotid artery, effective reduction in stroke risk throughout the procedure, and long-term restraint of carotid plaque. The Company commercialized its products in the United States in April 2016. Reverse Stock Split On March 13, 2019, the Company’s Board of Directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 2.7-for-1 reverse stock split of the Company’s common stock and redeemable convertible preferred stock. The par values of the common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. All common stock, redeemable convertible preferred stock, stock options and warrants, and related per share amounts in the condensed consolidated financial statements have been retroactively adjusted for all periods presented to give effect to the reverse stock split. The reverse stock split was effected on March 27, 2019. Initial Public Offering In April 2019, the Company issued and sold 6,000,000 shares of its common stock in its initial public offering (“IPO”) at a public offering price of $20.00 per share, for net proceeds of approximately $108,764,000 after deducting underwriting discounts and commissions of approximately $8,400,000 and expenses of approximately $2,836,000. Upon the closing of the IPO, all shares of convertible preferred stock then outstanding converted into shares of common stock and the Company’s outstanding warrants to purchase shares of common and convertible preferred stock were exercised, or automatically net exercised absent a prior election, resulting in the reclassification of the fair value of the related redeemable convertible preferred stock warrant liability to additional paid-in-capital.	Formation and Business of the Company The Company Silk Road Medical, Inc. (the “Company”) was incorporated in the state of Delaware on March 21, 2007. The Company has developed a technologically advanced, minimally-invasive solution for patients with carotid artery disease who are at risk for stroke. The Company’s portfolio of TCAR products enable a new procedure, referred to as transcarotid artery revascularization, or TCAR, that combines the benefits of endovascular techniques and surgical principles. The Company’s manufactures and sells in the United States its portfolio of TCAR products which are designed to provide direct access to the carotid artery, effective reduction in stroke risk throughout the procedure, and long-term restraint of carotid plaque. The Company commercialized its products in the United States in April 2016. Liquidity and Going Concern In the course of its activities, the Company has incurred losses and negative cash flows from operations since its inception. As of December 31, 2018, the Company had an accumulated deficit of $139,111,000. The Company expects to incur losses for the foreseeable future. The Company does not believe that its cash and cash equivalents of $24,990,000 at December 31, 2018, as well as its expected revenues and additional borrowings available under the loan agreement with CRG Partners III L.P. and certain of its affiliated funds (collectively “CRG”) will provide sufficient funds to allow the Company to fund its planned current operations for the next twelve months from the issuance of these consolidated financial statements. The Company expects to seek additional funding in the form of debt or equity financings to make strategic investments in its business; however, there can be no assurance that such efforts will be successful or that, in the event that they are successful, the terms and conditions of such financing will be favorable. If the Company’s revenue levels from its products are not sufficient or if the Company is unable to secure additional funding when desired, the Company may need to delay the development, commercialization and marketing of its products and scale back its business and operations. The Company has concluded that there is substantial doubt about its ability to continue as a going concern within one year after the date that the consolidated financial statements are issued. The accompanying consolidated financial statements do not include any adjustments that might result from the outcome of this uncertainty. Accordingly, the consolidated financial statements have been prepared on a basis that assumes the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business. Reverse Stock Split On March 13, 2019, the Company’s Board of Directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a 1-for-2.7 reverse stock split of the Company’s common stock and redeemable convertible preferred stock to be consummated prior to the effectiveness of the Company’s planned initial public offering (“IPO”). The par values of the common stock and redeemable convertible preferred stock were not adjusted as a result of the reverse stock split. Accordingly, all common stock, redeemable convertible preferred stock, stock options and warrants, and related per share amounts in the consolidated financial statements have been retroactively adjusted for all periods presented to give effect to the reverse stock split. The reverse stock split was effected on March 27, 2019.

Summary of Significant Accounting Policies

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Basis of Preparation

The accompanying financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America, or U.S. GAAP, and applicable rules and regulations of the Securities and Exchange Commission, or SEC, regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2018, and related disclosures, have been derived from the audited consolidated financial statements at that date but does not include all of the information required by GAAP for complete consolidated financial statements. These unaudited condensed consolidated financial statements have been prepared on the same basis as the Company’s annual consolidated financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for the fair statement of the Company’s condensed consolidated financial information. The results of

operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other interim period or for any other future year.

The accompanying interim unaudited condensed consolidated financial statements and related financial information should be read in conjunction with the audited consolidated financial statements and the related notes thereto for the year ended December 31, 2018 which are included elsewhere in the Company’s prospectus.

Principles of Consolidation

Through December 17, 2018, the condensed consolidated financial statements of the Company include the accounts of Silk Road Medical, Inc. and its consolidated variable interest entity (“VIE”), NeuroCo, Inc. On December 17, 2018, the Company acquired all assets and assumed all liabilities of its VIE. As a result of the Merger, NeuroCo merged into the Company with the Company being the surviving corporation. All intercompany balances and transactions have been eliminated in consolidation.

Pro Forma Balance Sheet Information

The pro forma consolidated balance sheet as of March 31, 2019 reflects: (i) the conversion of all outstanding shares of redeemable convertible preferred stock into shares of common stock immediately prior to the completion of the Company’s IPO; (ii) the cash and net exercise of the redeemable convertible preferred stock warrants into shares of common stock, based on the initial public offering price of $20.00 per share, and the related reclassification of the redeemable convertible warrant liability to common stock and additional paid-in-capital; and (iii) the cash and net exercise of the common stock warrants into shares of common stock, based on the initial public offering price of $20.00 per share. Shares of common stock sold in the IPO and contemplated to be sold in the Company’s planned public offering and related net proceeds are excluded from the pro forma information.

Use of Estimates

The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements. Management uses significant judgment when making estimates related to the common stock valuation and related stock-based compensation, the valuation of the redeemable convertible preferred stock warrants, the valuation of deferred tax assets, provisions for doubtful accounts receivable and excess and obsolete inventories, clinical trial accruals, and the reserves for sales returns. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Although these estimates are based on the Company’s knowledge of current events and actions it may undertake in the future, actual results may ultimately materially differ from these estimates and assumptions.

Fair Value of Financial Instruments

The Company has evaluated the estimated fair value of its financial instruments as of December 31, 2018 and March 31, 2019. The carrying amounts of cash equivalents, restricted cash, accounts receivable, accounts payable and accrued liabilities approximate their respective fair values because of the short-term nature of these instruments. Management believes that its borrowings bear interest at the prevailing market rates for instruments with similar characteristics; accordingly, the carrying value of this instrument

approximates its fair value. Fair value accounting is applied to the redeemable convertible preferred stock warrant liability.

Cash, Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with an original maturity of three months or less at the time of purchase to be cash equivalents. Cash equivalents are considered available-for-sale marketable securities and are recorded at fair value, based on quoted market prices. As of December 31, 2018 and March 31, 2019, the Company’s cash equivalents are entirely comprised of investments in money market funds.

Restricted cash as of December 31, 2018 and March 31, 2019 consists of a letter of credit of $310,000 representing collateral for the Company’s facility lease.

Concentration of Credit Risk, and Other Risks and Uncertainties

Financial instruments that potentially subject the Company to credit risk consist of cash and cash equivalents and accounts receivable to the extent of the amounts recorded on the condensed consolidated balance sheet.

The Company’s policy is to invest in money market funds, which are classified as cash equivalents on the condensed consolidated balance sheet. The Company’s cash are held in Company accounts at two financial institutions and such amounts may exceed federally insured limits. The Company’s money market funds are invested in highly rated money market funds.

The Company provides for uncollectible amounts when specific credit problems are identified. In doing so, the Company analyzes historical bad debt trends, customer credit worthiness, current economic trends and changes in customer payment patterns when evaluating the adequacy of the allowance for doubtful accounts.

The Company’s accounts receivable are due from a variety of health care organizations in the United States. At December 31, 2018 and March 31, 2019, no customer represented 10% or more of the Company’s accounts receivable. For the three months ended March 31, 2018 and March 31, 2019, there were no customers that represented 10% or more of revenue.

The Company manufactures certain of its commercial products in-house. Certain of the Company’s product components and sub-assemblies continue to be manufactured by sole suppliers, the most significant of which is the ENROUTE stent. Disruption in component or sub-assembly supply from these manufacturers or from in-house production would have a negative impact on the Company’s financial position and results of operations.

The Company is subject to certain risks, including that its devices may not be approved or cleared for marketing by governmental authorities or be successfully marketed. There can be no assurance that the Company’s products will achieve widespread adoption in the marketplace, nor can there be any assurance that existing devices or any future devices can be developed or manufactured at an acceptable cost and with appropriate performance characteristics. The Company is also subject to risks common to companies in the medical device industry, including, but not limited to, new technological innovations, dependence upon third-party payers to provide adequate coverage and reimbursement, dependence on key personnel and suppliers, protection of proprietary technology, product liability claims, and compliance with government regulations.

Existing or future devices developed by the Company may require approvals or clearances from the FDA or international regulatory agencies. In addition, in order to continue the Company’s operations, compliance with various federal and state laws is required. If the Company were denied or delayed in receiving such approvals or clearances, it may be necessary to adjust operations to align with the Company’s currently approved portfolio. If clearance for the products in the current portfolio were withdrawn by the FDA, this would have a material adverse impact on the Company.

Deferred Initial Public Offering Costs

Specific incremental legal, accounting and other fees and costs directly attributable to a proposed or actual offering of securities may properly be deferred and charged against the gross proceeds of the offering. In the event the Company’s planned IPO did not occur or was significantly delayed, all of the costs will be expensed. As of December 31, 2018 and March 31, 2019, there were $950,000 and $2,585,000, respectively, of initial public offering costs primarily consisting of legal and accounting fees that were capitalized in other non-current assets on the condensed consolidated balance sheet.

Leases

The Company adopted Accounting Standards Codification (“ASC”) 842, “Leases,” on January 1, 2019 and used the modified retrospective method for all leases not substantially completed as of the date of adoption and the package of practical expedients available in the standard. As a result of adopting ASC 842, the Company recorded an operating lease right-of-use (“ROU”) asset of $3,982,000 included within other non-current assets and operating lease liabilities of $5,190,000 included within accrued liabilities and other liabilities on the condensed consolidated balance sheet related to its facility lease, based on the present value of the future lease payments on the date of adoption. The operating lease right-of-use asset also includes adjustments for prepayments and excludes lease incentives. The adoption did not have an impact on prior periods or on its condensed consolidated statements of operations and comprehensive loss.

In accordance with ASC 842, the disclosure impact of adoption on the condensed consolidated balance sheet were as follows (in thousands):



Balance Sheet:	Balance at December 31, 2018		Adjustments Due to ASC 842		Balance at January 1, 2019
Other non-current assets	$	—	$	3,982	$	3,982
Accrued liabilities	139		582		721
Other liabilities	1,069		3,400		4,469

The Company recognizes ROU assets and lease liabilities when it obtains the right to control an asset under a leasing arrangement with an initial term greater than twelve months. The Company evaluates the nature of each lease at the inception of an arrangement to determine whether it is an operating or financing lease and recognizes the right-of-use asset and lease liabilities based on the present value of future minimum lease payments over the expected lease term. The Company’s leases do not generally contain an implicit interest rate and therefore the Company uses the incremental borrowing rate it would expect to pay to borrow on a similar collateralized basis over a similar term in order to determine the present value of its lease payments. The Company’s considers renewal options in the determination of the lease term if the option to renew is reasonably certain. The Company has elected to account

separately for contracts that contain lease and non-lease components consistent with its historical practice. Variable lease payments will be expensed as incurred.

Redeemable Convertible Preferred Stock Warrant Liability

The Company accounts for its warrants for shares of redeemable convertible preferred stock as a liability based upon the characteristics and provisions of each instrument. Redeemable convertible preferred stock warrants classified as a liability are initially recorded at their fair value on the date of issuance and are subject to remeasurement at each subsequent balance sheet date. Any change in fair value as a result of a remeasurement is recognized as a component of other income (expense), net in the condensed consolidated statements of operations and comprehensive loss.

Redeemable Convertible Preferred Stock

The Company records its redeemable convertible preferred stock at fair value on the dates of issuance, net of issuance costs. A redemption event will only occur upon the liquidation or winding up of the Company, a greater than 50% change in control, or sale of substantially all of the assets of the Company. In the event of a change of control of the Company, proceeds received from the sale of such shares will be distributed in accordance with the liquidation preferences set forth in the Company’s amended and restated certificate of incorporation unless the holders of redeemable convertible preferred stock otherwise agree or have converted their shares into shares of common stock. Therefore, redeemable convertible preferred stock is classified outside of stockholders’ deficit on the balance sheet as events triggering the liquidation preferences are not solely within the Company’s control. The Company is not required to adjust the carrying values of the redeemable convertible preferred stock to the redemption value of such shares since it is uncertain whether or when a redemption event will occur. Subsequent adjustments to increase the carrying values to the redemption values will be made only when it becomes probable that such redemption will occur.

Revenue Recognition

On January 1, 2018, the Company adopted ASC Topic 606, “Revenue from Contracts with Customers,” using the modified retrospective method applied to contracts which were not completed as of that date. Revenue for reporting periods beginning after January 1, 2018 are presented under ASC 606, while prior period revenue amounts are not adjusted and continue to be reported in accordance with the Company’s historic accounting under ASC 605, “Revenue Recognition.” Under ASC 606, revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps:


(i)	identify the contract(s) with a customer;


(ii)	identify the performance obligations in the contract;


(iii)	determine the transaction price;


(iv)	allocate the transaction price to the performance obligations in the contract; and


(v)	recognize revenue when (or as) the entity satisfies a performance obligation.

Under ASC 606, assuming all other revenue recognition criteria have been met, the Company will recognize revenue earlier for arrangements where the Company has satisfied its performance obligations

but have not issued invoices. As of December 31, 2018 and March 31, 2019, the Company recorded $128,000 and $131,000, respectively, of unbilled receivables, which are included in accounts receivable, net on the condensed consolidated balance sheet, as the Company has an unconditional right to payment as of the end of the applicable period.

The Company’s revenue is generated from the sale of its products to hospitals and medical centers in the United States through direct sales representatives. Revenue is recognized when obligations under the terms of a contract with customers are satisfied, which occurs with the transfer of control of the Company’s products to its customers, either upon shipment of the product or delivery of the product to the customer under the Company’s standard terms and conditions. The Company’s products are readily available for usage as soon as the customer possesses it. Upon receipt, the customer controls the economic benefits of the product, has significant risks and rewards, and the legal title. The Company has present right to payment; therefore, the transfer of control is deemed to happen at a point in time. Revenue is measured as the amount of consideration the Company expects to receive in exchange for transferring the goods.

For sales where the Company’s sales representative hand delivers product directly to the hospital or medical center from the sales representative’s trunk stock inventory, the Company recognizes revenue upon delivery, which represents the point in time when control transfers to the customer. Upon delivery there are legally-enforceable rights and obligations between the parties which can be identified, commercial substance exists and collectibility is probable. For sales which are sent directly from the Company to hospitals and medical centers, the transfer of control occurs at the time of shipment or delivery of the product. There are no further performance obligations by the Company or the sales representative to the customer after delivery under either method of sale. As allowed under the practical expedient, the Company does not disclose the value of unsatisfied performance obligations for (i) contracts with an original expected length of one year or less and (ii) contracts for which it recognizes revenue at the amount to which it has the right to invoice for services performed.

The Company is entitled to the total consideration for the products ordered by customers as product pricing is fixed according to the terms of customer contracts and payment terms are short. Payment terms fall within the one-year guidance for the practical expedient which allows the Company to forgo adjustment of the promised amount of consideration for the effects of a significant financing component. The Company excludes taxes assessed by governmental authorities on revenue-producing transactions from the measurement of the transaction price.

Costs associated with product sales include commissions and royalties. The Company applies the practical expedient and recognizes commissions and royalties as expense when incurred because the expense is incurred at a point in time and the amortization period is less than one year. Commissions are recorded as selling expense and royalties are recorded as cost of revenue in the condensed consolidated statements of operations and comprehensive loss.

The Company accepts product returns at its discretion or if the product is defective as manufactured. The Company establishes estimated provisions for returns based on historical experience. The Company elected to expense shipping and handling costs as incurred and includes them in the cost of goods sold. In those cases where the Company bills shipping and handling costs to customers, it will classify the amounts billed as a component of revenue.

Cost of Goods Sold

materials, components and subassemblies, manufacturing overhead costs, direct labor, reserves for excess, obsolete and non-sellable inventories as well as distribution-related expenses. A significant portion of the Company’s cost of goods sold currently consists of manufacturing overhead costs. These overhead costs include the cost of quality assurance, material procurement, inventory control, facilities, equipment and operations supervision and management. Cost of goods sold also includes depreciation expense for production equipment and certain direct costs such as shipping costs and royalties.

Stock–Based Compensation

The Company accounts for stock-based compensation in accordance with Financial Accounting Standards Board (“FASB”) ASC 718, “Compensation-Stock Compensation.” ASC 718 requires the recognition of compensation expense, using a fair-value based method, for costs related to all share-based payments including stock options. ASC 718 requires companies to estimate the fair value of all share-based payment option awards on the date of grant using an option pricing model. The fair value of stock options is recognized over the period during which an optionee is required to provide services in exchange for the option award, known as the requisite service period (usually the vesting period), on a straight-line basis. For performance-based stock options, the Company will assess the probability of performance conditions being achieved in each reporting period. The amount of stock-based compensation expense recognized in any one period related to performance-based stock options can vary based on the achievement or anticipated achievement of the performance conditions. The Company accounts for option forfeitures as they occur.

Income Taxes

The Company accounts for income taxes under the liability method, whereby deferred tax assets and liabilities are determined based on the difference between the condensed consolidated financial statements and tax bases of assets and liabilities using the enacted tax rates in effect for the year in which the differences are expected to affect taxable income. A valuation allowance is established when necessary to reduce deferred tax assets to the amounts expected to be realized. As the Company has historically incurred operating losses, it has established a full valuation allowance against its net deferred tax assets, and there is no provision for income taxes.

The Company also follows the provisions of ASC 740-10, “Accounting for Uncertainty in Income Taxes.” ASC 740-10 prescribes a comprehensive model for the recognition, measurement, presentation and disclosure in financial statements of any uncertain tax positions that have been taken or expected to be taken on a tax return. No liability related to uncertain tax positions is recorded on the condensed consolidated financial statements. It is the Company’s policy to include penalties and interest expense related to income taxes as part of the provision for income taxes.

Net Loss per Share Attributable to Common Stockholders

Basic net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration for potential dilutive common shares. Diluted net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock and warrants, and common stock options are considered to be potentially dilutive securities. Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share as the inclusion of all potential dilutive common shares would have been anti-dilutive.

The Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. The shares of the Company’s redeemable convertible preferred stock participate in any dividends declared by the Company and are therefore considered to be participating securities.

Net loss per share was determined as follows (in thousands, except share and per share data):



	Three Months Ended March 31,
	2018			2019
Net loss attributable to Silk Road Medical, Inc. common stockholders	$	(5,408	)	$	(24,158	)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	739,308			1,200,719
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted	$	(7.31	)	$	(20.12	)

The following potentially dilutive securities outstanding have been excluded from the computation of diluted weighted average shares outstanding because such securities have an antidilutive impact due to the Company’s net loss, in common stock equivalent shares:



	March 31,
	2018	2019
Redeemable convertible preferred stock outstanding	21,233,190	21,238,105
Redeemable convertible preferred stock warrants outstanding	2,672,502	2,667,587
Common stock options	4,431,797	4,138,635
Common stock warrants outstanding	—	7,527
	28,337,489	28,051,854

Pro Forma Net Loss per Share Attributable to Common Stockholders

The pro forma basic and diluted net loss per share has been computed to give effect to the conversion of the shares of redeemable convertible preferred stock into common stock immediately prior to the closing of the Company’s IPO, as if such conversion had occurred at the beginning of the period, and the cash and net exercise of the redeemable convertible preferred and common stock warrants, as if such exercise had occurred at the beginning of the period or the issuance date, if later. In addition, the numerator in the pro forma basic and diluted net loss per share calculation has been adjusted to remove the change in the fair value resulting from the remeasurement of the redeemable convertible preferred stock warrant liability as the redeemable convertible preferred stock warrants were cash and net exercised into common stock, and the related redeemable convertible preferred stock warrant liability reclassified to stockholders’ deficit immediately prior to the IPO closing. The denominator in the pro forma basic and diluted net loss per share calculation has been adjusted to include (i) the conversion of all outstanding shares of redeemable convertible preferred stock and (ii) the number of shares into which the redeemable convertible preferred stock warrants and common stock warrants were converted upon their cash or net exercise immediately prior to the closing of the IPO, based on the initial public offering price of $20.00 per share. The unaudited pro forma net loss per share does not include the shares of common stock contemplated to be sold in the Company’s planned public offering and related net proceeds.

Unaudited pro forma basic and diluted loss per share is computed as follows (in thousands, except share and per share data):



	Three Months Ended March 31,
	2019

Numerator:
Net loss and comprehensive loss attributable to Silk Road Medical, Inc. common stockholders	$	(24,158	)
Adjust: Change in fair value of redeemable convertible preferred stock warrants	15,712
Pro forma net loss	$	(8,446	)
Denominator:
Weighted average common shares used to compute net loss per share, basic and diluted	1,200,719
Adjust: Conversion of redeemable convertible preferred stock	21,233,190
Adjust: Cash and net exercise of redeemable convertible preferred stock warrants into common stock	1,941,105
Adjust: Cash and net exercise of common stock warrants into common stock	5,970
Weighted average common shares used to compute pro forma net loss per share, basic and diluted	24,380,984
Pro forma net loss per share, basic and diluted	$	(0.35	)

Comprehensive Loss

For the three months ended March 31, 2018 and March 31, 2019, there was no difference between comprehensive loss and the Company’s net loss.

Segment and Geographical Information

The Company operates and manages its business as one reportable and operating segment. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company’s long-lived assets are based in the United States. Long-lived assets are comprised of property and equipment. All of the Company’s revenue was in the United States for the three months ended March 31, 2018 and March 31, 2019, based on the shipping location of the external customer.

Summary of Significant Accounting Policies

Basis of Preparation

The accompanying consolidated financial statements have been prepared in accordance with the accounting principles generally accepted in the United States of America (“U.S. GAAP”). The consolidated financial statements include the accounts of the Company and its consolidated subsidiary. All intercompany accounts and transactions have been eliminated in consolidation.

Principles of Consolidation

As of December 31, 2017, the consolidated financial statements of the Company include the accounts of Silk Road Medical, Inc. and its consolidated variable interest entity (“VIE”). Disclosure regarding the Company’s participation in the VIE is included in Note 12, “Variable Interest Entity – NeuroCo”. On December 17, 2018, the Company acquired all assets and assumed all liabilities of its VIE. All intercompany balances and transactions have been eliminated in consolidation.

Variable Interest Entity

As of December 31, 2017, the Company had an interest in a VIE. Determining whether to consolidate a VIE requires judgment in assessing (i) whether an entity is a VIE and (ii) if the Company is the entity’s primary beneficiary and thus required to consolidate the entity. To determine if the Company is the primary beneficiary of a VIE, the Company evaluates whether it has (i) the power to direct the activities that most significantly impact the VIE’s economic performance and (ii) the obligation to absorb losses or the right to receive benefits of the VIE that could potentially be significant to the VIE. The Company’s evaluation includes identification of significant activities and an assessment of its ability to direct those activities.

Use of Estimates

Fair Value of Financial Instruments

The Company has evaluated the estimated fair value of its financial instruments as of December 31, 2017 and 2018. The carrying amounts of cash equivalents, restricted cash, accounts receivable, accounts payable and accrued liabilities approximate their respective fair values because of the short-term nature of these instruments. Management believes that its borrowings bear interest at the prevailing market rates for instruments with similar characteristics; accordingly, the carrying value of this instrument approximates its fair value. Fair value accounting is applied to the redeemable convertible preferred stock warrant liability.

Cash, Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with an original maturity of three months or less at the time of purchase to be cash equivalents. Cash equivalents are considered available-for-sale marketable securities and are recorded at fair value, based on quoted market prices. As of December 31, 2017 and 2018, the Company’s cash equivalents are entirely comprised of investments in money market funds.

The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total of the same amounts shown in the consolidated statements of cash flows (in thousands):



	December 31,
	2017		2018
Cash and cash equivalents	$	33,331	$	24,990
Restricted cash	510		310
Total cash, cash equivalents and restricted cash	$	33,841	$	25,300

Restricted cash as of December 31, 2017 and 2018 consists of a letter of credit of $310,000 representing collateral for the Company’s facility lease. As of December 31, 2017, restricted cash additionally included a certificate of deposit of $200,000 associated with the Company’s corporate credit cards.

Concentration of Credit Risk, and Other Risks and Uncertainties

The Company’s policy is to invest in money market funds, which are classified as cash equivalents on the consolidated balance sheet. The Company’s cash are held in Company accounts at two financial institutions and such amounts may exceed federally insured limits. The Company’s money market funds are invested in highly rated money market funds.

The Company’s accounts receivable are due from a variety of health care organizations in the United States. At December 31, 2017 and 2018, no customer represented 10% or more of the Company’s accounts receivable. For the years ended December 31, 2017 and 2018, there were no customers that represented 10% or more of revenue.

Accounts Receivable

Trade accounts receivable are recorded at the invoiced amount and do not bear interest. The Company estimates allowances for doubtful accounts and for product returns. Specifically, the Company makes estimates on the collectability of customer accounts and sales returns and allowances based primarily on analysis of historical trends and experience and changes in customers’ financial condition. The Company uses its judgment, based on the best available facts and circumstances, and records an allowance against amounts due to reduce the receivable to the amount that is expected to be collected. These specific allowances are reevaluated and adjusted as additional information is received that impacts the amount reserved. To date, the Company has not experienced material credit-related losses.

Inventories

Inventories are valued at the lower of cost to purchase or manufacture the inventory or net realizable value. Cost is determined using the first-in, first-out method for all inventories. Net realizable value is determined as the estimated selling prices in the ordinary course of business, less reasonably predictable costs of completion, disposal and transportation. The Company regularly reviews inventory quantities in consideration of actual loss experiences, projected future demand, and remaining shelf life to record a provision for excess and obsolete inventory when appropriate. The Company’s policy is to write down inventory that has become obsolete, inventory that has a cost basis in excess of its expected lower of cost or net realizable value, and inventory in excess of expected requirements. The estimate of excess quantities is judgmental and primarily dependent on the Company’s estimates of future demand for a particular product. If the estimate of future demand is too high, the Company may have to increase the reserve for excess inventory for that product and record a charge to the cost of goods sold.

Property and Equipment

Property and equipment are recorded at cost less accumulated depreciation or amortization. Repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated using the straight-line method over the estimated useful lives of the assets, typically three to five years. Leasehold improvements are amortized using the straight-line method over the shorter of the lease term or estimated useful economic life of the asset. When assets are retired or otherwise disposed of, the cost and accumulated depreciation are removed from the balance sheet and any resulting gain or loss is reflected in operations in the period realized.

Deferred Initial Public Offering Costs

Specific incremental legal, accounting and other fees and costs directly attributable to a proposed or actual offering of securities may properly be deferred and charged against the gross proceeds of the offering. In the event the Company’s planned IPO does not occur or is significantly delayed, all of the costs will be expensed. As of December 31, 2018 there were $950,000 of offering costs primarily consisting of legal and accounting fees that were capitalized in other non-current assets on the consolidated balance sheet. No deferred offering costs were capitalized as of December 31, 2017.

Impairment of Long-Lived Assets

The Company reviews long-lived assets, including property and equipment, for impairment whenever events or changes in business circumstances indicate that the carrying amount of the assets may not be fully recoverable. If indicators of impairment exist, an impairment loss would be recognized when estimated undiscounted future cash flows expected to result from the use of the assets and their eventual disposition are less than their carrying amount. Impairment, if any, is measured as the amount by which the carrying amount of the long-lived assets exceeds their fair value. The Company did not record any impairment of long-lived assets for the years ended December 31, 2017 and 2018.

Redeemable Convertible Preferred Stock Warrant Liability

The Company accounts for its warrants for shares of redeemable convertible preferred stock as a liability based upon the characteristics and provisions of each instrument. Redeemable convertible preferred stock warrants classified as a liability are initially recorded at their fair value on the date of issuance and are subject to remeasurement at each subsequent balance sheet date. Any change in fair value as a result of a remeasurement is recognized as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss.

Redeemable Convertible Preferred Stock

Revenue Recognition

On January 1, 2018, the Company adopted Accounting Standards Codification (“ASC”) Topic 606, “Revenue from Contracts with Customers,” using the modified retrospective method applied to contracts which were not completed as of that date. Revenue for the year ended December 31, 2018 is presented under ASC 606, while prior period revenue amounts are not adjusted and continue to be reported in accordance with the Company’s historic accounting under ASC 605, “Revenue Recognition.” Under ASC 606, revenue is recognized when a customer obtains control of promised goods or services,

in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. To determine revenue recognition for arrangements that an entity determines are within the scope of ASC 606, the Company performs the following five steps:


(i)	identify the contract(s) with a customer;


(ii)	identify the performance obligations in the contract;


(iii)	determine the transaction price;


(iv)	allocate the transaction price to the performance obligations in the contract; and


(v)	recognize revenue when (or as) the entity satisfies a performance obligation.

Under ASC 606, assuming all other revenue recognition criteria have been met, the Company will recognize revenue earlier for arrangements where the Company has satisfied its performance obligations but have not issued invoices. As of December 31, 2018, the Company recorded $128,000 of unbilled receivables, which are included in accounts receivable, net on the consolidated balance sheet, as the Company has an unconditional right to payment as of the end of the applicable period.

The Company recognized the cumulative effect of initially applying ASC 606 as an adjustment to the opening balance of accumulated deficit. The cumulative effect of the changes made to the consolidated balance sheet as of January 1, 2018 for the adoption of ASC 606 were as follows (in thousands):



	Balance at December 31, 2017			Adjustments Due to ASC 606			Balance at January 1, 2018
Accounts receivable, net	$	5,215		$	136		$	5,351
Inventories	3,248			(46		)	3,202
Accrued liabilities	3,109			4			3,113
Accumulated deficit	(101,556		)	87			(101,469		)

In accordance with ASC 606, the disclosure of the impact of adoption on the consolidated balance sheet and statement of operations and comprehensive loss were as follows (in thousands):



	Year Ended December 31, 2018
	Balance As Reported			Balance Before ASC 606 Adoption			Effect of Change
Balance sheet:
Accounts receivable, net	$	4,520		$	4,494		$	26
Inventories	5,744			5,766			(22		)
Accrued liabilities	7,586			7,586			—
Accumulated deficit	(139,111		)	(139,107		)	(4		)
Statement of operations and comprehensive loss:
Revenue	34,557			34,583			(26		)
Cost of goods sold	10,874			10,852			22

Costs associated with product sales include commissions and royalties. The Company applies the practical expedient and recognizes commissions and royalties as expense when incurred because the expense is incurred at a point in time and the amortization period is less than one year. Commissions are recorded as selling expense and royalties are recorded as cost of revenue in the consolidated statements of operations and comprehensive loss.

As noted, revenue for the year ended December 31, 2018 is presented under ASC 606, while prior period revenue amounts are not adjusted and continue to be reported in accordance with the Company’s historic accounting under ASC 605, “Revenue Recognition.” Under ASC 605, the Company recognized revenue when all of the following criteria were met:


•	persuasive evidence of an arrangement exists;


•	the sales price is fixed or determinable;


•	collection of the relevant receivable is reasonably assured at the time of sale; and


•	delivery has occurred or services have been rendered.

The Company recognized revenue when title to the goods and risk of loss transferred to the customer, which was upon shipment of the product under the Company’s standard terms and conditions. The Company estimated reductions in revenue for potential returns of products by customers. In making such estimates, management analyzed historical returns, current economic trends and changes in customer demand and acceptance of its products. The Company expensed shipping and handling costs as incurred and included them in the cost of goods sold. In those cases where the Company billed shipping and handling costs to customers, it would classify the amounts billed as a component of revenue.

Cost of Goods Sold

The Company manufactures certain of its portfolio of TCAR products at its facility and purchases other products from third party manufacturers. Cost of goods sold consists primarily of costs related to materials, components and subassemblies, manufacturing overhead costs, direct labor, reserves for excess, obsolete and non-sellable inventories as well as distribution-related expenses. A significant portion of the Company’s cost of goods sold currently consists of manufacturing overhead costs. These overhead costs include the cost of quality assurance, material procurement, inventory control, facilities, equipment and operations supervision and management. Cost of goods sold also includes depreciation expense for production equipment and certain direct costs such as shipping costs and royalties.

Research and Development

The Company expenses research and development costs as incurred. Research and development expenses consist primarily of engineering, product development, clinical studies to develop and support the Company’s products, regulatory expenses, medical affairs and other costs associated with products and technologies that are in development. Research and development expenses include employee compensation, including stock-based compensation, supplies, consulting, prototyping, testing, materials, travel expenses, depreciation and an allocation of facility overhead expenses. Additionally, research and development expenses include costs associated with our clinical studies including clinical trial design, clinical site reimbursement, data management, travel expenses and the cost of products used for clinical trials and internal and external costs associated with the Company’s regulatory compliance and quality assurance functions, including the costs of outside consultants and contractors that assist in the process of submitting and maintaining regulatory filings, and overhead costs.

Clinical Trials

The Company accrues and expenses costs for its clinical trial activities performed by third parties, including clinical research organizations and other service providers, based upon estimates of the work completed over the life of the individual study in accordance with associated agreements. The Company determines these estimates through discussion with internal personnel and outside service providers as to progress or stage of completion of trials or services pursuant to contracts with clinical research organizations and other service providers and the agreed-upon fee to be paid for such services.

Advertising Costs

The Company expenses advertising costs as incurred. Advertising costs include design and production costs, including website development, physician and patient testimonial videos, written media campaigns, and other items. Advertising costs of $218,000 and $186,000 were expensed during the years ended December 31, 2017 and 2018, respectively.

Foreign Currency

The Company records net gains and losses resulting from foreign exchange transactions as a component of foreign currency exchange gains or losses in other income (expense), net. The Company had no material foreign currency exchange gains or losses during the years ended December 31, 2017 and 2018.

Stock–Based Compensation

The Company accounts for stock-based compensation in accordance with Financial Accounting Standards Board (“FASB”) Accounting Standards Codification (“ASC”) 718, “Compensation-Stock Compensation.” ASC 718 requires the recognition of compensation expense, using a fair-value based method, for costs related to all share-based payments including stock options. ASC 718 requires companies to estimate the fair value of all share-based payment option awards on the date of grant using an option pricing model. The fair value of stock options is recognized over the period during which an optionee is required to provide services in exchange for the option award, known as the requisite service period (usually the vesting period), on a straight-line basis. For performance-based stock options, the Company will assess the probability of performance conditions being achieved in each reporting period. The amount of stock-based compensation expense recognized in any one period related to performance-based stock options can vary based on the achievement or anticipated achievement of the performance conditions.

In March 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-09, “Stock Compensation (Topic 718): Improvements to Employee Shared-Based Payment Accounting.” Under ASU 2016-09, entities are permitted to make an accounting policy election to either estimate forfeitures on share-based payment awards, as previously required, or to recognize forfeitures as they occur. The Company made an accounting policy election to account for forfeitures as they occur. This change has been applied on a modified retrospective basis, resulting in a cumulative-effect adjustment to increase accumulated deficit by $13,000 as of January 1, 2018, the date of adoption.

Prior to January 1, 2018, the Company accounted for equity instruments issued to nonemployees in accordance with ASC 505-50 “Accounting for Equity Instruments that are Issued to Other than Employees for Acquiring, or in Conjunction with Selling Goods or Services.” Equity instruments issued to nonemployees were recorded at their fair value on the measurement date and were subject to periodic adjustments as the underlying equity instruments vest. The Company believed that the fair value of the equity instrument was more reliably measured than the fair value of the services received.

Income Taxes

The Company accounts for income taxes under the liability method, whereby deferred tax assets and liabilities are determined based on the difference between the consolidated financial statements and tax bases of assets and liabilities using the enacted tax rates in effect for the year in which the differences are expected to affect taxable income. A valuation allowance is established when necessary to reduce deferred tax assets to the amounts expected to be realized.

The Company also follows the provisions of ASC 740-10, “Accounting for Uncertainty in Income Taxes.” ASC 740-10 prescribes a comprehensive model for the recognition, measurement, presentation and disclosure in financial statements of any uncertain tax positions that have been taken or expected to be taken on a tax return. No liability related to uncertain tax positions is recorded on the consolidated financial statements. It is the Company’s policy to include penalties and interest expense related to income taxes as part of the provision for income taxes.

Net Loss per Share Attributable to Common Stockholders

Basic net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock outstanding during the period, without consideration for potential dilutive common shares. Diluted net loss per share is computed by dividing the net loss attributable to common stockholders by the weighted average number of shares of common stock and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock and warrants, and common stock options are considered to be potentially dilutive securities. Since the Company was in a loss position for all periods presented, basic net loss per share is the same as diluted net loss per share as the inclusion of all potential dilutive common shares would have been anti-dilutive.

Net loss per share was determined as follows (in thousands, except share and per share data):



	Year Ended December 31,
	2017			2018
Net loss attributable to Silk Road Medical, Inc. common stockholders	$	(19,356	)	$	(37,629	)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	434,158			960,882
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted	$	(44.58	)	$	(39.16	)



	December 31,
	2017	2018
Redeemable convertible preferred stock outstanding	21,233,190	21,233,190
Redeemable convertible preferred stock warrants outstanding	2,672,502	2,672,502
Common stock options	4,308,890	4,364,377
Common stock warrants outstanding	—	7,527
	28,214,582	28,277,596

Unaudited Pro Forma Net Loss per Share Attributable to Common Stockholders

The unaudited pro forma basic and diluted net loss per share has been computed to give effect to the conversion of the shares of redeemable convertible preferred stock into common stock immediately prior to the closing of the Company’s IPO, as if such conversion had occurred at the beginning of the period, and both the cash and net exercise of the redeemable convertible preferred and common stock warrants, as if such exercise had occurred at the beginning of the period or the issuance date, if later. In addition, the numerator in the pro forma basic and diluted net loss per share calculation has been adjusted to remove the change in the fair value resulting from the remeasurement of the redeemable convertible

preferred stock warrant liability as the redeemable convertible preferred stock warrants were cash or net exercised into common stock, and the related redeemable convertible preferred stock warrant liability will be reclassified to stockholders’ deficit immediately prior to the closing of the IPO. The denominator in the pro forma basic and diluted net loss per share calculation has been adjusted to include (i) the conversion of all outstanding shares of redeemable convertible preferred stock and (ii) the number of shares into which the redeemable convertible preferred stock warrants and common stock warrants were converted upon their cash or net exercise immediately prior to the closing of the IPO, based on the initial public offering price of $20.00 per share. The unaudited pro forma net loss per share does not include the shares to be sold and related proceeds received from the IPO.

Unaudited pro forma basic and diluted loss per share is computed as follows (in thousands, except share and per share data):



	Year Ended December 31,
	2018
	(unaudited)
Numerator:
Net loss and comprehensive loss attributable to Silk Road Medical, Inc. common stockholders	$	(37,629	)
Adjust: Change in fair value of redeemable convertible preferred stock warrants	11,906
Pro forma net loss	$	(25,723	)
Denominator:
Weighted average common shares used to compute net loss per share, basic and diluted	960,882
Adjust: Conversion of redeemable convertible preferred stock	21,233,190
Adjust: Cash and net exercise of redeemable convertible preferred stock warrants into common stock warrants	1,940,450
Adjust: Cash and net exercise of common stock warrants into common stock	246
Weighted average common shares used to compute pro forma net loss per share, basic and diluted	24,134,768
Pro forma net loss per share, basic and diluted	$	(1.07	)

Comprehensive Loss

For the years ended December 31, 2017 and 2018, there was no difference between comprehensive loss and the Company’s net loss.

Segment and Geographical Information

The Company operates and manages its business as one reportable and operating segment. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company’s long-lived assets are based in the United States. Long-lived assets are comprised of property and equipment. All of the Company’s revenue was in the United States for the years ended December 31, 2017 and 2018, based on the shipping location of the external customer.

Recent Accounting Pronouncements	3 Months Ended
Recent Accounting Pronouncements	Mar. 31, 2019
Accounting Changes and Error Corrections [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Standards In the first quarter of 2018, the Company adopted ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), and its associated amendments. Under the standard, revenue is recognized when a customer obtains control of promised goods or services in an amount that reflects the consideration the entity expects to receive in exchange for those goods or services. In addition, the standard requires disclosure of the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The Company applied the five step method outlined in the ASU to all revenue streams and elected to utilize the modified retrospective implementation method. The additional disclosures required by the ASU have been included in Note 2, “Summary of Significant Accounting Policies.” In the first quarter of 2018, the Company adopted ASU No. 2016-18, Statement of Cash Flows: Restricted Cash, a consensus of the Financial Accounting Standards Board (“FASB”) Emerging Issues Task Force. Under the standard, restricted cash and restricted cash equivalent amounts are presented within cash and cash equivalents when reconciling the total beginning and ending amounts shown on the statement of cash flows. When cash, cash equivalents, restricted cash and restricted cash equivalents are presented in more than one line item on the balance sheet, a reconciliation of the totals in the statement of cash flows to the related captions in the balance sheet is required. The impact of the adoption of ASU No. 2016-18 resulted in a decrease in investing activities of $310,000 and an increase in the ending cash, cash equivalents and restricted cash of $510,000 in the consolidated statement of cash flows for the year ended December 31, 2017. The impact of the adoption resulted in a decrease in investing activities and an increase in the ending cash, cash equivalents and restricted cash of $200,000 in the consolidated statement of cash flows for the year ended December 31, 2018. In the first quarter of 2018, the Company adopted ASU No. 2017-09, Compensation - Stock Compensation - Scope of Modification Accounting. The standard provides clarification on when modification accounting should be used for changes to the terms or conditions of a share-based payment award. This standard does not change the accounting for modifications but clarifies that modification accounting guidance should only be applied if there is a change to the value, vesting conditions, or award classification and would not be required if the changes are considered non-substantive. The guidance was adopted on a prospective basis in the first quarter of 2018 and did not have any impact upon adoption. In the first quarter of 2018, the Company adopted ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting. ASU 2018-07 simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. Adoption of this guidance did not have a material impact on the Company’s consolidated financial statements. Recently Issued Accounting Standards In February 2016, the FASB issued ASU 2016-02, Leases (“ASC 842”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASC 842 supersedes the previous leases standard, ASC 840, Leases. For public entities, the standard is effective for interim and annual periods beginning after December 15, 2018 and should be applied through a modified retrospective transition approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with early adoption permitted. The Company plans to adopt the new standard on January 1, 2019 and elect the optional transition method. The Company will also elect the package of transitional practical expedients such that the Company will retain lease classification and initial direct costs for leases existing prior to the adoption of the new lease standard. The Company will also elect the hindsight practical expedient. Although the Company is currently evaluating the impact of this guidance on its consolidated financial statements, it does expect that most of its operating lease commitments will be subject to the new guidance and will be recognized as operating lease liabilities and right-of-use assets upon its adoption. In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Statements. This update provides financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The update replaces the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU No. 2016-13 is effective for public entities for annual periods beginning after December 15, 2019. The Company does not believe that the adoption of this new guidance will have a material impact on its consolidated financial statements. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement, which changed the disclosure requirements for fair value measurements by removing, adding and modifying certain disclosures. The standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, Cloud Computing Arrangements, which aligns the requirements for capitalizing implementation costs in a Cloud Computing Arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. The standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures. Recent Accounting Pronouncements Recently Adopted Accounting Standards In February 2016, the FASB issued ASU No. 2016-02, “Leases,” that supersedes Accounting Standards Codification (“ASC”) 840, “Leases.” Subsequently, the FASB issued several updates to ASU No. 2016-02, codified in ASC Topic 842 (“ASC 842”). The Company adopted ASC 842 on January 1, 2019 using the modified retrospective method for all leases not substantially completed as of the date of adoption. The Company elected to apply the package of practical expedients, which allowed the Company to not reassess: (i) whether expired or existing contracts contain leases; (ii) lease classification for any expired or existing leases; and (iii) initial direct costs for any existing lease. Recently Issued Accounting Standards In June 2016, the FASB issued Accounting Standards Update (“ASU”) 2016-13, “Measurement of Credit Losses on Financial Statements.” This update provides financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The update replaces the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU No. 2016-13 is effective for public entities for annual periods beginning after December 15, 2019. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-13, “Fair Value Measurement,” which changed the disclosure requirements for fair value measurements by removing, adding and modifying certain disclosures. The standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-15, “Cloud Computing Arrangements,” which aligns the requirements for capitalizing implementation costs in a Cloud Computing Arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. The standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures.

Fair Value Measurements

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

The Company measures certain financial assets and liabilities at fair value on a recurring basis. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or a liability. A three-tier fair value hierarchy is established as a basis for considering such assumptions and for inputs used in the valuation methodologies in measuring fair value:


•	Level 1 – quoted prices in active markets are identical assets and liabilities;


•	Level 2 – observable inputs other than quotes prices in active markets for identical assets and liabilities;


•	Level 3 – unobservable inputs.

The Company’s cash equivalents are classified within Level 1 of the fair value hierarchy because they are valued using quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency.

In August 2014 through April 2016, the Company issued warrants to purchase 2,672,502 shares of Series C redeemable convertible preferred stock at the exercise price of $6.11 per share. As a derivative liability, the redeemable convertible warrants were initially recorded at fair value and are subject to remeasurement at each balance sheet date. Any change in fair value as a result of a remeasurement is recognized as a component of other income (expense), net in the condensed consolidated statements of operations and comprehensive loss. The Company’s redeemable convertible warrant liability is classified within Level 3 of the fair value hierarchy.

At December 31, 2018, the fair value of the redeemable convertible warrant liability was determined by using an option pricing model to allocate the total enterprise value to the various securities within the Company’s capital structure. As of December 31, 2018, the fair value of the redeemable convertible warrant liability was based on both the estimated fair value of the Company’s common stock and on valuation models discounted at current implied market rates which are based on Level 3 inputs. Additionally, the model’s inputs reflect assumptions that market participants would use in pricing the instrument in a current period transaction and included:



	December 31,
	2018
Time to liquidity (years)	0.57
Expected volatility	62.5%
Discounted cash flow rate	12.0%
Risk-free interest rate	2.6%
Marketability discount rate	14%

At March 31, 2019, due to the proximity of the Company’s IPO, the fair value of the redeemable convertible warrant liability was based on the estimated fair value of the Company’s common stock.

The following table sets forth the fair value of the Company’s financial liabilities measured on a recurring basis, as of December 31, 2018 and March 31, 2019 (in thousands):



	December 31, 2018
	Level 1		Level 2		Level 3		Total
Liabilities
Redeemable convertible warrant liability	$	—	$	—	$	16,091	$	16,091



	March 31, 2019
	Level 1		Level 2		Level 3		Total
Liabilities
Redeemable convertible warrant liability	$	—	$	—	$	31,803	$	31,803

The changes in the redeemable convertible warrant liability are summarized below (in thousands):



Fair Value at December 31, 2018	$	16,091
Change in fair value recorded in other income (expense), net	15,712
Fair Value at March 31, 2019	$	31,803

There were no transfers between fair value hierarchy levels during the three months ended March 31, 2018 and 2019.

Fair Value Measurements


•	Level 1 – quoted prices in active markets are identical assets and liabilities;


•	Level 2 – observable inputs other than quotes prizes in active markets for identical assets and liabilities;


•	Level 3 – unobservable inputs.

In October and December 2015, the Company issued warrants to purchase 1,395,468 and 5,324 shares, respectively, of Series C redeemable convertible preferred stock at the exercise price of $6.11 per share. The Company recorded an initial warrant liability of $4,879,000. The redeemable convertible warrant liability was initially valued using the Black Scholes option-pricing valuation method with the following assumptions: a remaining contractual term of 8 years, a volatility of 52%, and a risk-free interest rate of 1.94% for the warrants issued in October, and a remaining contractual term of 8 years, a volatility of 52%, and a risk-free interest rate of 2.24% for the warrants issued in December. In April 2016, the Company issued additional warrants to purchase 42,608 shares of Series C redeemable convertible preferred stock at the exercise price of $6.11 per share. The Company recorded a warrant liability of $144,000. The redeemable convertible warrant liability was initially valued using the Black Scholes option-pricing valuation method with the following assumptions: a remaining contractual term of 7.5 years, a volatility of 50%, and a risk-free interest rate of 1.64%.

As a derivative liability, the redeemable convertible warrants were initially recorded at fair value and are subject to remeasurement at each balance sheet date. Any change in fair value as a result of a remeasurement is recognized as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss. The Company’s redeemable convertible warrant liability is classified within Level 3 of the fair value hierarchy.

At December 31, 2017 and 2018, the fair value of the redeemable convertible warrant liability was determined by using an option pricing model to allocate the total enterprise value to the various securities within the Company’s capital structure. The fair value of the redeemable convertible warrant liability was based on both the estimated fair value of the Company’s common stock of $4.78 and $11.29 as of December 31, 2017 and 2018, respectively, and on valuation models discounted at current implied market rates which are based on Level 3 inputs. Additionally, the model’s inputs reflect assumptions that market participants would use in pricing the instrument in a current period transaction and included:



	Year Ended December 31,
	2017	2018
Time to liquidity (years)	1.75	0.57
Expected volatility	50.0%	62.5%
Discounted cash flow rate	18.0%	12.0%
Risk-free interest rate	1.9%	2.6%
Marketability discount rate	27%	14%

The following table sets forth the fair value of the Company’s financial liabilities measured on a recurring basis, as of December 31, 2017 and 2018 (in thousands):



	December 31, 2017
	Level 1		Level 2		Level 3		Total
Liabilities
Redeemable convertible warrant liability	$	—	$	—	$	4,185	$	4,185



	December 31, 2018
	Level 1		Level 2		Level 3		Total
Liabilities
Redeemable convertible warrant liability	$	—	$	—	$	16,091	$	16,091

The changes in the redeemable convertible warrant liability are summarized below (in thousands):



Fair value at December 31, 2016	$	7,143
Change in fair value recorded in other income (expense), net	(2,958		)
Fair value at December 31, 2017	4,185
Change in fair value recorded in other income (expense), net	11,906
Fair value at December 31, 2018	$	16,091

There were no transfers between fair value hierarchy levels during the years ended December 31, 2017 and 2018.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Balance Sheet Components

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Balance Sheet Components

Inventories



(in thousands)	December 31,		March 31,
	2018		2019
Raw materials	$	1,054	$	2,147
Finished products	4,690		5,909
	$	5,744	$	8,056

As of December 31, 2018 and March 31, 2019, there were no work-in-process inventories.

Balance Sheet Components

Inventories



(in thousands)	December 31,
	2017		2018
Raw materials	$	506	$	1,054
Finished products	2,742		4,690
	$	3,248	$	5,744

As of December 31, 2017 and 2018, there were no work-in-process inventories.

Property and Equipment, Net



(in thousands)	December 31,
	2017			2018
Furniture and fixtures	$	76		$	517
Equipment	1,059			1,217
Software	405			76
Leasehold improvements	189			1,978
	1,729			3,788
Less: Accumulated depreciation and amortization	(1,303		)	(946		)
Add: Construction-in-progress	60			38
	$	486		$	2,880

Depreciation and amortization expense was $129,000 and $517,000 for the years ended December 31, 2017 and 2018, respectively.

Accrued Liabilities



(in thousands)	December 31,
	2017		2018
Accrued payroll and related expenses	$	2,718	$	5,157
Accrued professional services	64		1,014
Accrued royalty expense	—		313
Accrued travel expenses	68		270
Accrued clinical expenses	183		244
Accrued other expenses	76		588
	$	3,109	$	7,586

Long-term Debt

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Debt Disclosure [Abstract]

Long-term Debt

In October 2015, the Company entered into a term loan agreement with CRG. The term loan agreement provides for up to $30,000,000 in term loans split into two tranches as follows: (i) the Tranche A Loans provided for $20,000,000 in term loans, and (ii) the Tranche B Loans provided for up to $10,000,000 in term loans. The Company drew down the Tranche A Loans on October 13, 2015. The Tranche B Loans were available to be drawn prior to March 29, 2017. In January 2017, the term loan agreement was amended to extend the commitment period of the Tranche B Loans to April 28, 2017. In April 2017, the Company drew down $5,000,000 of the available Tranche B Loans.

In September 2018, the Company entered into Amendment No. 5 to the term loan agreement with CRG. Under the amended terms of the amended loan agreement the maturity date was extended to December 31, 2022 and the repayment schedule of the existing term loans were changed to interest only so that the outstanding principal amount of the term loans will be payable in a single installment at maturity. The related fixed interest rate was changed to equal 10.75% per annum, due and payable quarterly in arrears. At the election of the Company, 2.75% of the interest due and payable may be “paid in kind”, or PIK, and added to the then outstanding principal and 8.0% of the interest due and payable paid in cash. All unpaid principal, and accrued and unpaid interest, is due and payable in full on December 31, 2022. The amended term loan agreement also provided for additional term loans in an aggregate principal amount of up to $25,000,000. In September 2018, the Company drew down an additional $15,000,000 under the term loan agreement with CRG.

The Company may voluntarily prepay the borrowings in full, with a prepayment premium beginning at 8.0% and declining to 4.0% after the fourth payment date, to 2.0% after the eighth payment date, with no premium being payable if prepayment occurs after the third year of the loan. The Tranche A borrowing required a payment, on the borrowing date, of a financing fee equal to 1.75% of the borrowed loan principal, which is recorded as a discount to the debt. In addition, a facility fee equal to 5.0% of the amounts borrowed plus any PIK is payable at the end of the term or when the borrowings are repaid in full. A long-term liability is being accreted using the effective interest method for the facility fee over the term of the loan agreement. The borrowings are collateralized by a security interest in substantially all of the Company’s assets.

The Company is subject to financial covenants related to liquidity and minimum trailing revenue targets that begin in December 31, 2016 and are tested on an annual basis. The liquidity covenant requires the Company to maintain an amount which shall exceed the greater of (i) $3,000,000 and (ii) the minimum cash balance, if any, required of the Company by a creditor to the extent the Company has incurred permitted priority debt. The Company had to achieve minimum net revenue of $1,000,000 in 2016,

$5,000,000 in 2017, $15,000,000 in 2018, and must achieve minimum net revenue of $30,000,000 in 2019 and $40,000,000 in 2020. The liquidity financial covenant has a 90-day equity cure period following end of the calendar year to issue additional shares of equity interests in exchange for cash, or to borrow permitted cure debt. In addition, the term loan agreement prohibits the payment of cash dividends on the Company’s capital stock and also places restrictions on mergers, sales of assets, investments, incurrence of liens, incurrence of indebtedness and transactions with affiliates. CRG may accelerate the payment terms of the term loan agreement upon the occurrence of certain events of default set forth therein, which include the failure of the Company to make timely payments of amounts due under the term loan agreement, the failure of the Company to adhere to the covenants set forth in the term loan agreement, the insolvency of the Company or upon the occurrence of a material adverse change. As of March 31, 2019, the Company was in compliance with all applicable financial covenants. As of March 31, 2019, management does not believe that it is probable that the above clauses will be triggered within the next twelve months, and therefore, the debt is classified as long-term on the condensed consolidated balance sheet.

The issuance costs and debt discount have been netted against the borrowed funds on the condensed consolidated balance sheet. The long-term debt balance as of March 31, 2019 was $44,597,000.

Future maturities under the term loan agreement as of March 31, 2019 are as follows (in thousands):



Period Ending December 31:	Amount
2019	$	2,696
2020	3,677
2021	3,771
2022	52,510
	62,654
Add: Accretion of closing fees	958
	63,612
Less: Amount representing interest	(18,841		)
Less: Amount representing debt discount and debt issuance costs	(174		)
Present value of minimum payments	$	44,597

In October 2015, CRG purchased 327,759 shares of the Company’s Series C redeemable convertible preferred stock at $6.11 per share. In addition, CRG received warrants to purchase 163,877 shares of the Company’s Series C redeemable convertible preferred stock. The warrants are immediately exercisable, at an exercise price per share of $6.11, and expire the earlier of October 2023 or upon the consummation of a change of control or initial public offering of the Company.

In July 2017, CRG purchased 163,877 shares of the Company’s Series C convertible preferred stock at $6.11 per share.

Long-term Debt

In September 2018, the Company entered into Amendment No. 5 to the term loan agreement with CRG. Under the amended terms of the amended loan agreement the maturity date was extended to December 31, 2022 and the repayment schedule of the existing term loans were changed to interest only so that the outstanding principal amount of the term loans will be payable in a single installment at maturity. The related fixed interest rate was changed to equal 10.75% per annum, due and payable quarterly in arrears. At the election of the Company, 2.75% of the interest due and payable may be “paid in kind” and added to the then outstanding principal and 8.0% of the interest due and payable paid in cash. All unpaid principal, and accrued and unpaid interest, is due and payable in full on December 31, 2022. The amended term loan agreement also provided for additional term loans in an aggregate principal amount of up to $25,000,000 and allow for the conversion into shares of common stock, at the Company’s option, of up to 25% of the outstanding loans under the term loan agreement in connection with an initial public offering of the Company’s common stock which results in market capitalization of at least $250,000,000. In September 2018, the Company drew down an additional $15,000,000 under the term loan agreement with CRG.

The Company may voluntarily prepay the borrowings in full, with a prepayment premium beginning at 8.0% and declining to 4.0% after the fourth payment date, to 2.0% after the eighth payment date, with no premium being payable if prepayment occurs after the third year of the loan. The Tranche A borrowing required a payment, on the borrowing date, of a financing fee equal to 1.75% of the borrowed loan principal, which is recorded as a discount to the debt. In addition, a facility fee equal to 5.0% of the amounts borrowed plus any “paid in kind” is payable at the end of the term or when the borrowings are repaid in full. A long-term liability is being accreted using the effective interest method for the facility fee over the term of the loan agreement. The borrowings are collateralized by a security interest in substantially all of the Company’s assets.

cash balance, if any, required of the Company by a creditor to the extent the Company has incurred permitted priority debt. The Company had to achieve minimum net revenue of $1,000,000 in 2016, $5,000,000 in 2017, $15,000,000 in 2018, and must achieve minimum net revenue of $30,000,000 in 2019 and $40,000,000 in 2020. The liquidity financial covenant has a 90-day equity cure period following end of the calendar year to issue additional shares of equity interests in exchange for cash, or to borrow permitted cure debt. In addition, the term loan agreement prohibits the payment of cash dividends on the Company’s capital stock and also places restrictions on mergers, sales of assets, investments, incurrence of liens, incurrence of indebtedness and transactions with affiliates. CRG may accelerate the payment terms of the term loan agreement upon the occurrence of certain events of default set forth therein, which include the failure of the Company to make timely payments of amounts due under the term loan agreement, the failure of the Company to adhere to the covenants set forth in the term loan agreement, the insolvency of the Company or upon the occurrence of a material adverse change. As of December 31, 2018, the Company was in compliance with all applicable financial covenants. As of December 31, 2018, management does not believe that it is probable that the above clauses will be triggered within the next twelve months, and therefore, the debt is classified as long-term on the consolidated balance sheet.

The issuance costs and debt discount have been netted against the borrowed funds on the consolidated balance sheet. The long-term debt balance as of December 31, 2018 was $44,201,000.

Future maturities under the term loan agreement as of December 31, 2018 are as follows (in thousands):



Year Ending December 31:	Amount
2019	$	3,566
2020	3,677
2021	3,771
2022	52,510
	63,524
Add: Accretion of closing fees	872
	64,396
Less: Amount representing interest	(20,010		)
Less: Amount representing debt discount and debt issuance costs	(185		)
Present value of minimum payments	$	44,201

In July 2017, CRG purchased 163,877 shares of the Company’s Series C convertible preferred stock at $6.11 per share.

Commitments and Contingencies

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

Operating Lease and Rights of Use

The Company’s operating lease obligation consists of leased office, laboratory, and manufacturing space under a non-cancellable operating lease that expires in October 2024. Operating lease costs were $217,000 for the three months ended March 31, 2019. Cash paid for amounts included in the

measurement of operating lease liabilities was $193,000 for the three months ended March 31, 2019. As of March 31, 2019, the weighted average discount rate was approximately 6.50% and the weighted average remaining lease term was 5.60 years. Balance sheet information as of March 31, 2019 consists of the following (in thousands):



Operating Lease:	March 31, 2019
Operating lease right-of-use asset in other non-current assets	$	3,820
Operating lease liability in accrued liabilities	$	712
Operating lease liability in other liabilities	4,285
Total operating lease liabilities	$	4,997

The following table summarizes the Company’s operating lease maturities as of March 31, 2019 (in thousands):



Period Ending December 31:	Amount
2019	$	760
2020	1,037
2021	1,066
2022	1,096
2023	1,127
2024	904
Total lease payments	5,990
Less: imputed interest	(993		)
Present value of lease liabilities	$	4,997

Minimum future lease payments previously disclosed under ASC 840 in the Company’s audited consolidated financial statements and the related notes thereto for the year ended December 31, 2018 which are included elsewhere in the Company’s prospectus are as follows (in thousands):



Year Ending December 31:	Total Minimum Lease Payments
2019	$	1,002
2020	1,002
2021	1,031
2022	1,044
2023	1,920
	$	5,999

Purchase Obligations

Purchase obligations consist of agreements to purchase goods and services entered into in the ordinary course of business. The Company had non-cancellable commitments for inventory that were payable within one year to suppliers for purchases totaling $2,855,100 as of March 31, 2019.

Indemnification

In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and may provide for indemnification of the counterparty. The Company’s exposure under these agreements is unknown because it involves claims that may be made against it in the future but have not yet been made. To date, the Company has not been subject to any claims or been required to defend any action related to its indemnification obligations.

The Company indemnifies each of its directors and officers for certain events or occurrences, subject to certain limits, while the director is or was serving at the Company’s request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director in such capacity. The maximum amount of potential future indemnification is unlimited; however, the Company currently holds director liability insurance. This insurance allows the transfer of risk associated with the Company’s exposure and may enable it to recover a portion of any future amounts paid. The Company believes that the fair value of these indemnification obligations is minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations as of March 31, 2019.

Contingencies

The Company is not involved in any pending legal proceedings that it believes could have a material adverse effect on its financial condition, results of operations or cash flows. From time to time, the Company may pursue litigation to assert its legal right and such litigation may be costly and divert the efforts and attention of its management and technical personnel which could adversely affect its business. The Company accrues a liability for such matters when it is probable that future expenditures will be made and such expenditures can be reasonably estimated. There were no contingent liabilities requiring accrual at December 31, 2018 and March 31, 2019.

Legal Matters

In February 2019, a former employee, through counsel, advised the Company that he had filed a charge of discrimination against the Company with the California Department of Fair Employment & Housing, or DFEH. The former employee’s complaint alleges sexual harassment and retaliation in violation of the California Department of Fair Employment & Housing Act. The complaint does not allege specific damages. To date, the DFEH has not contacted the Company. The Company denies the complaint’s allegations and intends to vigorously defend itself. At this time the Company cannot estimate the outcomes or possible loss or range of loss arising from this claim, if any; as such, no accrual was included in the Company’s balance sheet as of March 31, 2019.

Commitments and Contingencies

Operating Leases

The Company’s operating lease obligations primarily consist of leased office, laboratory, and manufacturing space under non-cancellable operating leases that expire in January 2019 and in

October 2024. In November 2017, the Company entered into a six-year operating lease for new office space in Sunnyvale, the lease commenced in June 2018 and expires in October 2024. The lease agreement includes a renewal provision allowing the Company to extend this lease for an additional period of five years. In addition to the minimum future lease commitments presented below, the lease requires the Company to pay property taxes, insurance, maintenance, and repair costs. The lease includes a rent holiday concession and escalation clauses for increased rent over the lease term. Rent expense is recognized using the straight-line method over the term of the lease. The Company records deferred rent calculated as the difference between rent expense and the cash rental payments. In connection with the facility lease, the landlord provided incentives of $794,000 to the Company in the form of leasehold improvements. In addition, the landlord also provided for leasehold improvements financing of $316,000. The financing amount was added to the Company’s minimum lease commitments as of the lease commencement date at an interest rate of 7.0% per annum. These amounts have been reflected as deferred rent and are being amortized as a reduction to rent expense over the original term of the Company’s operating lease.

The aggregate future minimum lease payments as of December 31, 2018 are as follows (in thousands):



Year Ending December 31:	Total Minimum Lease Payments
2019	$	1,002
2020	1,002
2021	1,031
2022	1,044
2023 and thereafter	1,920
	$	5,999

Purchase Obligations

Indemnification

minimal. Accordingly, the Company has not recognized any liabilities relating to these obligations as of December 31, 2018.

Legal Proceedings

Redeemable Convertible Preferred Stock

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Redeemable Convertible Preferred Stock

The Company has the following redeemable convertible preferred stock issued and outstanding at December 31, 2018:



	December 31, 2018
	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,227,992	$	6.11	74,713		59,129
	24,069,615	21,233,190			$	121,144	$	105,235

The Company has the following redeemable convertible preferred stock issued and outstanding at March 31, 2019:



	March 31, 2019
	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,232,907	$	6.11	74,743		59,159
	24,069,615	21,238,105			$	121,174	$	105,265

As of December 31, 2018 and March 31, 2019, the holders of redeemable convertible preferred stock (“convertible preferred stock”) have various rights and preferences as follows:

Voting Rights

The holders of Series A, Series A-1, Series B and Series C convertible preferred stock are entitled to vote on all matters on which the common stockholders are entitled to vote. Holders of Series A, Series A-1, Series B and Series C convertible preferred and common stock vote together as a single class. Each holder of Series A, Series A-1, Series B and Series C convertible preferred stock is entitled to the number of votes equal to the number of common stock shares into which the shares held by such holder are convertible.

Election of Directors

The holders of record of Series A and Series B preferred stock, exclusively and as a separate class, are entitled to each elect two and three directors of the Company, and the holders of record of Series C preferred stock, exclusively and as a separate class, are entitled to elect two directors of the Company.

Dividends

The holders of Series A, Series A-1, Series B and Series C convertible preferred stock are entitled, on a pari passu basis, when and if declared by the Board of Directors of the Company, to non-cumulative dividends out of the Company’s assets legally available therefore at the rate of $0.22, $0.27, $0.49 and $0.49 per share per annum, respectively. No distributions will be made with respect to the common stock until all declared but unpaid dividends on convertible preferred stock have been paid or set aside for payment to the convertible preferred stock holders. The right to receive dividends on shares of convertible preferred stock will be non-cumulative, and no right to such dividends will accrue to holders of convertible preferred stock by reason of the fact that dividends on such shares are not declared or paid in any years. As of March 31, 2019, no dividends have been declared to date.

Liquidation

In the event of any liquidation, dissolution or winding up of the Company, either voluntary or involuntary, the holders of Series C preferred stock will be entitled to receive out of net available funds and assets of the corporation available for distribution to its stockholders, before any payment shall be made to the holders of Series B preferred stock and Series A preferred stock and Series A-1 junior stock. After the payment of all preferential amounts required to be paid to the holders of Series C preferred stock, the holders of Series B, Series A and Series A-1 outstanding shares of convertible preferred stock will be entitled to receive out of net available funds and assets, before and in preference to any distribution of any of the Company’s net available funds and assets to the holders of common stock by reason of their ownership of such common stock.

An amount per share equal to $6.11, $6.11, $3.38 and $2.70 for each share of Series C, Series B, Series A-1 and Series A, respectively, convertible preferred stock then so held equal to the applicable liquidation preference. The remaining assets, if any, shall be distributed to the holders of common stock. Should the Company’s legally available assets be insufficient to satisfy the liquidation preferences after the payment of all preferential amounts required to be paid to the holders of Series C preferred stock, the funds will be distributed ratably among the holders of Series A, Series A-1, and Series B convertible preferred stock in proportion to the preferential amount each holder is otherwise entitled to receive.

Conversion

Shares of convertible preferred stock are convertible into shares of common stock at the holders’ option at any time or automatically (i) immediately prior to the closing of a firmly underwritten public offering in which the offering price per share is not less than $18.31 and the aggregate gross proceeds received by the Company are not less than $50,000,000 or (ii) upon receipt by the Company of a written request for such conversion from the holders of the majority of the convertible preferred stock then outstanding, voting as a single class and on an as-converted basis. Each share of Series A, Series A-1, Series B and Series C convertible preferred stock is convertible, at the option of the holder, into the number of shares of common stock into which such shares are convertible at the then effective conversion ratio. The initial conversion price per share for Series A, Series A-1, Series B and Series C convertible preferred stock is $2.70, $3.38, $6.11 and $6.11 per share, respectively. The initial conversion price is subject to

adjustment from time to time. As of March 31, 2019, the conversion ratio for each series of convertible preferred stock was one-for-one.

Redemption

The redeemable convertible preferred stock is recorded in mezzanine equity because while it is not mandatorily redeemable, it will become redeemable at the option of the stockholders upon the occurrence of certain deemed liquidation events that are considered not solely within the Company’s control.

Preferred Stock Warrants

In connection with the issuance of the Company’s Series C redeemable convertible preferred stock issuances between August 2014 through April 2016, the Company issued, to each investor who purchased shares of Series C redeemable convertible preferred stock, warrants to purchase up to the number of shares of preferred stock equal to 50% of the number of shares of the Company’s Series C redeemable convertible preferred stock purchased.

The warrants are immediately exercisable, at an exercise price per share of $6.11 and expire 8 years from their date of issuance. The warrants will be automatically net exercised upon the consummation or effective date of a change of control or initial public offering of the Company.

As of December 31, 2018 and March 31, 2019, warrants to purchase an aggregate of 2,672,502 and 2,667,587, respectively, shares of Series C redeemable convertible preferred stock were outstanding.

Common Stock

At March 31, 2019, the Company’s certificate of incorporation, as amended and restated, authorizes the Company to issue up to 29,879,220 shares of common stock with $0.001 par value per share, of which 1,386,615 shares were issued and outstanding. The holders of common stock are also entitled to receive dividends whenever funds are legally available, when and if declared by the Board of Directors. As of March 31, 2019, no dividends have been declared to date. Each share of common stock is entitled to one vote.

At December 31, 2018 and March 31, 2019, the Company had reserved common stock for future issuances as follows:



	December 31,	March 31,
	2018	2019
Conversion of Series A convertible preferred stock	1,629,629	1,629,629
Conversion of Series A-1 convertible preferred stock	1,111,111	1,111,111
Conversion of Series B convertible preferred stock	6,264,470	6,264,470
Conversion of Series C convertible preferred stock and warrants	15,064,405	15,064,405
Exercise of options under stock plan	4,364,377	4,138,635
Issuance of options under stock plan	57,889	32,326
Warrants to purchase common stock	7,527	7,527
	28,499,408	28,248,103

Common Stock Warrants

In connection with the Company’s acquisition of NeuroCo in December 2018, as of the merger closing, the outstanding warrants to purchase common stock of NeuroCo converted to warrants to purchase 7,527 shares of the Company’s common stock.

The warrants are exercisable, at an exercise price per share of $8.27 and expire on November 21, 2024. The warrants will be automatically net exercised upon the consummation or effective date of a change of control or initial public offering of the Company.

As of December 31, 2018 and March 31, 2019, warrants to purchase an aggregate of 7,527 shares of common stock were outstanding.

Redeemable Convertible Preferred Stock

The Company has the following redeemable convertible preferred stock issued and outstanding at December 31, 2017 and 2018:



	December 31, 2017 and 2018
	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,227,992	$	6.11	74,713		59,129
	24,069,615	21,233,190			$	121,144	$	105,235

As of December 31, 2017 and 2018, the holders of redeemable convertible preferred stock (“convertible preferred stock”) have various rights and preferences as follows:

Voting Rights

Election of Directors

Dividends

until all declared but unpaid dividends on convertible preferred stock have been paid or set aside for payment to the convertible preferred stock holders. The right to receive dividends on shares of convertible preferred stock will be non-cumulative, and no right to such dividends will accrue to holders of convertible preferred stock by reason of the fact that dividends on such shares are not declared or paid in any years. As of December 31, 2018, no dividends have been declared to date.

Liquidation

Conversion

Redemption

Preferred Stock Warrants

number of shares of preferred stock equal to 50% of the number of shares of the Company’s Series C redeemable convertible preferred stock purchased.

The warrants are immediately exercisable, at an exercise price per share of $6.11 and expire eight years from their date of issuance. The warrants will be automatically net exercised upon the consummation or effective date of a change of control or initial public offering of the Company.

As of December 31, 2017 and 2018, warrants to purchase an aggregate of 2,672,502 shares of Series C redeemable convertible preferred stock were outstanding.

Common Stock

At December 31, 2018, the Company’s certificate of incorporation, as amended and restated, authorizes the Company to issue up to 29,879,220 shares of common stock with $0.001 par value per share, of which 1,135,310 shares were issued and outstanding. The holders of common stock are also entitled to receive dividends whenever funds are legally available, when and if declared by the Board of Directors. As of December 31, 2018, no dividends have been declared to date. Each share of common stock is entitled to one vote.

At December 31, 2017 and 2018, the Company had reserved common stock for future issuances as follows:



	December 31,
	2017	2018
Conversion of Series A convertible preferred stock	1,629,629	1,629,629
Conversion of Series A-1 convertible preferred stock	1,111,111	1,111,111
Conversion of Series B convertible preferred stock	6,264,470	6,264,470
Conversion of Series C convertible preferred stock and warrants	15,064,405	15,064,405
Exercise of options under stock plan	4,308,890	4,364,377
Issuance of options under stock plan	328,290	57,889
Warrants to purchase common stock	—	7,527
	28,706,795	28,499,408

Common Stock Warrants

As of December 31, 2018, warrants to purchase an aggregate of 7,527 shares of common stock were outstanding.

Common Stock

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Equity [Abstract]

Common Stock

Redeemable Convertible Preferred Stock

The Company has the following redeemable convertible preferred stock issued and outstanding at December 31, 2018:



	December 31, 2018
	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,227,992	$	6.11	74,713		59,129
	24,069,615	21,233,190			$	121,144	$	105,235

The Company has the following redeemable convertible preferred stock issued and outstanding at March 31, 2019:



	March 31, 2019
	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,232,907	$	6.11	74,743		59,159
	24,069,615	21,238,105			$	121,174	$	105,265

As of December 31, 2018 and March 31, 2019, the holders of redeemable convertible preferred stock (“convertible preferred stock”) have various rights and preferences as follows:

Voting Rights

Election of Directors

Dividends

Liquidation

Conversion

adjustment from time to time. As of March 31, 2019, the conversion ratio for each series of convertible preferred stock was one-for-one.

Redemption

Preferred Stock Warrants

As of December 31, 2018 and March 31, 2019, warrants to purchase an aggregate of 2,672,502 and 2,667,587, respectively, shares of Series C redeemable convertible preferred stock were outstanding.

Common Stock

At December 31, 2018 and March 31, 2019, the Company had reserved common stock for future issuances as follows:



	December 31,	March 31,
	2018	2019
Conversion of Series A convertible preferred stock	1,629,629	1,629,629
Conversion of Series A-1 convertible preferred stock	1,111,111	1,111,111
Conversion of Series B convertible preferred stock	6,264,470	6,264,470
Conversion of Series C convertible preferred stock and warrants	15,064,405	15,064,405
Exercise of options under stock plan	4,364,377	4,138,635
Issuance of options under stock plan	57,889	32,326
Warrants to purchase common stock	7,527	7,527
	28,499,408	28,248,103

Common Stock Warrants

As of December 31, 2018 and March 31, 2019, warrants to purchase an aggregate of 7,527 shares of common stock were outstanding.

Redeemable Convertible Preferred Stock

The Company has the following redeemable convertible preferred stock issued and outstanding at December 31, 2017 and 2018:



	December 31, 2017 and 2018
	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,227,992	$	6.11	74,713		59,129
	24,069,615	21,233,190			$	121,144	$	105,235

As of December 31, 2017 and 2018, the holders of redeemable convertible preferred stock (“convertible preferred stock”) have various rights and preferences as follows:

Voting Rights

Election of Directors

Dividends

Liquidation

Conversion

Redemption

Preferred Stock Warrants

number of shares of preferred stock equal to 50% of the number of shares of the Company’s Series C redeemable convertible preferred stock purchased.

As of December 31, 2017 and 2018, warrants to purchase an aggregate of 2,672,502 shares of Series C redeemable convertible preferred stock were outstanding.

Common Stock

At December 31, 2017 and 2018, the Company had reserved common stock for future issuances as follows:



	December 31,
	2017	2018
Conversion of Series A convertible preferred stock	1,629,629	1,629,629
Conversion of Series A-1 convertible preferred stock	1,111,111	1,111,111
Conversion of Series B convertible preferred stock	6,264,470	6,264,470
Conversion of Series C convertible preferred stock and warrants	15,064,405	15,064,405
Exercise of options under stock plan	4,308,890	4,364,377
Issuance of options under stock plan	328,290	57,889
Warrants to purchase common stock	—	7,527
	28,706,795	28,499,408

Common Stock Warrants

As of December 31, 2018, warrants to purchase an aggregate of 7,527 shares of common stock were outstanding.

Stock Option Plan

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock Option Plan

Stock Option Plans

In 2007, the Company established its 2007 Stock Option Plan (the “Plan”) which provides for the granting of stock options to employees, directors and consultants of the Company. Options granted under the Plan may be either incentive stock options (“ISOs”) or nonqualified stock options (“NSOs”). ISOs may be granted only to Company employees (including officers and directors who are also employees). NSOs may be granted to Company employees, directors and consultants. As of March 31, 2019, the Company has reserved 5,488,229 shares of common stock for issuance under the Plan.

In connection with its acquisition of NeuroCo in December 2018, the Company also assumed NeuroCo’s 2015 Equity Incentive Plan, or the NeuroCo Plan. As of the merger closing, the outstanding options to purchase common stock of NeuroCo under the NeuroCo Plan converted to options to purchase 1,442 shares of the Company’s common stock. There are no additional shares of common stock reserved for issuance under the NeuroCo Plan.

The exercise price of ISOs and NSOs shall not be less than 100% and 85% of the estimated fair value of the shares on the date of grant, respectively, as determined by the Board of Directors. The exercise price of ISOs and NSOs granted to a 10% stockholder shall not be less than 110% of the estimated fair value of the shares on the date of grant as determined by the Board of Directors. To date, options have a term of 10 years and generally vest over 4 years with 25% vesting on the first anniversary of the issuance date, and then monthly vesting for an additional 3 years from date of grant.

Activity under the Company’s Plan is set forth below:



			Options Outstanding
	Shares Available for Grant		Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value (in thousands)
Balances, December 31, 2018	57,889		4,364,377		$	3.79	7.36	$	33,132
Authorized	—
Options granted	(32,950	)	32,950		$	11.29
Options exercised	—		(251,305	)	$	1.50
Options cancelled	7,387		(7,387	)	$	1.56
Balances, March 31, 2019	32,326		4,138,635		$	4.00	7.26	$	30,602
Vested and exercisable at March 31, 2019			2,451,044		$	2.70	6.36	$	21,195
Vested and expected to vest at March 31, 2019			4,138,635		$	4.00	7.26	$	30,602

The aggregate intrinsic value of options exercised during the three months ended March 31, 2019 was $2,461,000. The aggregate intrinsic value was calculated as the difference between the exercise prices of the underlying options and the estimated fair value of the common stock on the date of exercise.

The Company’s Board of Directors approved the termination of the 2007 Stock Plan and the NeuroCo 2015 Equity Incentive Plan and the adoption of the 2019 Equity Incentive Plan, or the 2019 Plan, effective immediately prior to consummation of the Company’s IPO. The 2019 Plan provides for the grant of ISOs to employees and for the grant of NSOs, restricted stock, restricted stock units, stock appreciation rights, performance units and performance shares to employees, directors and consultants. A total of 2,317,000 shares of common stock were reserved for issuance pursuant to the 2019 Plan. In addition, the shares reserved for issuance under the 2019 Plan will also include shares reserved but not issued under the 2019 Plan, plus any share awards granted under the 2007 Plan that expire or terminate without having been exercised in full or that are forfeited or repurchased. In addition, the number of shares available for issuance under the 2019 Plan will also include an annual increase on the first day of each fiscal year beginning in fiscal 2020, equal to the lesser of (i) 3,000,000 shares; (ii) 4.0% of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year; or (iii) an amount as determined by the Board of Directors.

2019 Employee Stock Purchase Plan

In March 2019, the Company’s Board of Directors adopted the 2019 Employee Stock Purchase Plan (“ESPP”) under which eligible employees are permitted to purchase common stock at a discount through payroll deductions. A total of 434,000 shares of common stock are reserved for issuance and will be increased on the first day of each fiscal year, beginning in 2019, by an amount equal to the lesser of (i) 1,200,000 shares (ii) 1.0% of the outstanding shares of common stock as of the last day of the immediately preceding fiscal year; or (iii) an amount as determined by the Board of Directors. The price of the common stock purchased will be the lower of 85% of the fair market value of the common stock at the beginning of an offering period or at the end of a purchase period. The ESPP was effective upon adoption by the Company’s Board of Directors but was not in use until the completion of the IPO. The

ESPP is intended to qualify as an “employee stock purchase plan” within the meaning of Section 423 of the Internal Revenue Code of 1986, as amended.

Stock‑Based Compensation

The Company estimated the fair value of stock options using the Black–Scholes option pricing model. The fair value of employee stock options is being amortized on a straight–line basis over the requisite service period of the awards. The fair value of employee stock options was estimated using the following assumptions for the three months ended March 31, 2018 and 2019:



	Three Months Ended March 31,
	2018	2019
Expected term (in years)	6.25	6.25
Expected volatility	38.1%	42.7%
Risk-free interest rate	2.68%	2.54%
Dividend yield	—%	—%

Total stock-based compensation expense relating to the Company’s stock options to employees and nonemployees during the three months ended March 31, 2018 and 2019, is as follows (in thousands):



	Three Months Ended March 31,
	2018		2019
Cost of goods sold	$	13	$	15
Research and development expenses	29		27
Selling, general and administrative expenses	120		220
	$	162	$	262

As of March 31, 2019, there was total unrecognized compensation costs of $2,433,000 related to these stock options. These costs are expected to be recognized over a period of approximately 3.04 years.

Stock Option Plan

Plan may be either incentive stock options (“ISOs”) or nonqualified stock options (“NSOs”). ISOs may be granted only to Company employees (including officers and directors who are also employees). NSOs may be granted to Company employees, directors and consultants. As of December 31, 2018, the Company has reserved 5,488,229 shares of common stock for issuance under the Plan.

The exercise price of ISOs and NSOs shall not be less than 100% and 85% of the estimated fair value of the shares on the date of grant, respectively, as determined by the Board of Directors. The exercise price of ISOs and NSOs granted to a 10% stockholder shall not be less than 110% of the estimated fair value of the shares on the date of grant as determined by the Board of Directors. To date, options have a term of ten years and generally vest over 4 years with 25% vesting on the first anniversary of the issuance date, and then monthly vesting for an additional three years from date of grant.

Activity under the Company’s Plan and the NeuroCo Plan is set forth below:



			Options Outstanding
	Shares Available for Grant		Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value (in thousands)
Balances, December 31, 2016	27,605		3,478,346		$	1.46	7.71	$	444
Authorized	1,421,867
Options granted	(1,375,329	)	1,375,329		$	6.54
Options exercised	—		(290,638	)	$	1.17
Options cancelled	254,147		(254,147	)	$	1.55
Balances, December 31, 2017	328,290		4,308,890		$	3.09	7.81	$	5,073
Authorized	223,664
Options granted	(629,716	)	629,716		$	6.51
Options exercised	—		(438,578	)	$	1.50
Options cancelled	135,651		(135,651	)	$	1.56
Balances December 31, 2018	57,889		4,364,377		$	3.79	7.36	$	33,132
Vested and exercisable at December 31, 2018			2,459,116		$	2.35	6.28	$	22,109
Vested and expected to vest at December 31, 2018			4,464,377		$	3.79	7.36	$	33,132

The following table summarizes information about stock options outstanding at December 31, 2018:



	Options Outstanding and Vested as of December 31, 2018
	Options Outstanding				Options Vested
Exercise Price	Options Outstanding	Weighted Average Remaining Contractual Term (in Years)	Weighted Average Exercise Price		Number Exercisable	Weighted Average Exercise Price
$0.68	101,848	1.59	$	0.68	101,848	$	0.68
$1.35	107,938	2.75	$	1.35	107,938	$	1.35
$1.38	304,396	3.96	$	1.38	304,396	$	1.38
$1.46	385,070	6.21	$	1.46	360,470	$	1.46
$1.57	97,311	8.07	$	1.57	42,013	$	1.57
$1.60	1,463,632	6.82	$	1.60	1,128,569	$	1.60
$3.16	462,764	8.95	$	3.16	162,549	$	3.16
$4.73	332,385	8.92	$	4.73	86,506	$	4.73
$6.11	552,462	9.32	$	6.11	29,848	$	6.11
$8.27	78,941	9.93	$	8.27	1,527	$	8.27
$12.15	477,630	8.92	$	12.15	133,452	$	12.15
	4,364,377	7.36	$	3.79	2,459,116	$	2.35

Stock‑Based Compensation Associated with Awards to Employees

During the years ended December 31, 2017 and 2018, the Company granted stock options to employees to purchase 1,371,626 and 575,314 shares of common stock, with a weighted‑average grant date fair value of $0.83 and $2.81 per share, respectively. The total fair value of options vested during the years ended December 31, 2017 and 2018 was $423,000 and $569,000, respectively. The aggregate intrinsic value of options exercised was $143,000 and $787,000 during the years ended December 31, 2017 and 2018, respectively. The aggregate intrinsic value was calculated as the difference between the exercise prices of the underlying options and the estimated fair value of the common stock on the date of exercise. Stock‑based compensation expense recognized during the years ended December 31, 2017 and 2018 includes compensation expense for stock–based awards granted to employees based on the grant date fair value of $442,000 and $728,000, respectively.

The Company also issues stock options with vesting based upon completion of performance goals. The fair value for these performance-based awards is recognized over the period during which the goals are to be achieved. Stock-based compensation expense recognized at fair value includes the impact of estimated probability that the goals would be achieved, which is assessed prior to the requisite service period for the specific goals.



	Year Ended December 31,
	2017	2018
Expected term (in years)	1.00 - 6.25	6.25
Expected volatility	39% - 41%	38.1% - 38.8%
Risk-free interest rate	1.03% - 2.25%	2.68% - 2.98%
Dividend yield	—%	—%

The fair value of common stock was determined by the Company’s Board of Directors, who considered, among other things, contemporaneous valuations of the Company’s common stock prepared by an unrelated third-party valuation firm in accordance with the guidance provided by the American Institute of Certified Public Accountants Practice Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation. The expected term of stock options represents the weighted-average period the stock options are expected to remain outstanding. The Company does not have sufficient historical exercise and post-vesting termination activity to provide accurate data for estimating the expected term of options and has opted to use the “simplified method,” whereby the expected term equals the arithmetic average of the vesting term and the original contractual term of the option. The expected stock price volatility assumption was determined by examining the historical volatilities for industry peers, as the Company did not have any trading history for the Company’s common stock. The Company will continue to analyze the historical stock price volatility and expected term assumption as more historical data for the Company’s common stock becomes available. The risk-free interest rate assumption is based on the U.S. Treasury instruments whose term was consistent with the expected term of the Company’s stock options. The expected dividend assumption is based on the Company’s history and expectation of dividend payouts.

Effective January 1, 2018, the Company made an accounting policy election to account for forfeitures as they occur.

Stock-Based Compensation Associated with Awards to Nonemployees

During the years ended December 31, 2017 and 2018, the Company granted options to purchase 3,703 and 52,960 shares, respectively, of common stock to consultants in exchange for services. Stock–based compensation expense related to stock options granted to nonemployees is recognized as the stock options are earned.

The fair value of stock options granted to nonemployees was calculated using the following assumptions:



	Year Ended December 31,
	2017	2018
Contractual term (in years)	3.75 - 9.75	5.00 - 6.25
Expected volatility	39% - 56%	38.0% - 38.8%
Risk-free interest rate	2.06% - 2.39%	2.71% - 2.90%
Dividend yield	—%	—%

In connection with the grant of stock options to nonemployees, the Company recorded stock–based compensation charges of $93,000 and $183,000 for the years ended December 31, 2017 and 2018, respectively.

Total stock-based compensation expense relating to the Company’s stock options to employees and nonemployees during the years ended December 31, 2017 and 2018, is as follows (in thousands):



	Year Ended December 31,
	2017		2018
Cost of goods sold	$	49	$	51
Research and development expenses	98		256
Selling, general and administrative expenses	388		604
	$	535	$	911

As of December 31, 2018, there was total unrecognized compensation costs of $2,524,000 related to these stock options. These costs are expected to be recognized over a period of approximately 3.47 years.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

The Company uses the liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. The Company must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to the Company’s lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance.

The Company recognizes benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. The Company’s policy is to recognize interest and penalties related to the underpayment of income taxes as a component of income tax expense or benefit. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.

The components of income before taxes are as follows (in thousands):



	Year Ended December 31,
	2017			2018
United States	$	(22,358	)	$	(37,630	)
International	—			—
	$	(22,358	)	$	(37,630	)

A reconciliation of the statutory U.S. federal rate to the Company’s effective tax rate is as follows (in thousands):



	Year Ended December 31,
	2017			2018
Tax at federal statutory rate	$	(7,602	)	$	(7,902	)
State taxes, net of federal benefit	(1,155		)	(1,582		)
Permanent differences	535			289
Loss on Series C warrant liability	—			2,500
Tax Cut and Jobs Act	12,456			—
Change in valuation allowance	(3,832		)	6,197
General business credits	(465		)	136
Other	69			376
Provision for taxes	$	6		$	14

Significant components of the Company’s net deferred tax assets as of December 31, 2017 and 2018 consist of the following (in thousands):



	December 31,
	2017			2018
Deferred tax assets:
Net operating loss carryforwards	$	28,056		$	33,815
Research and development credits	5,081			4,944
Capitalized start-up costs/Intangibles	19			16
Accruals and reserves	705			1,169
Property and equipment	44			82
Stock-based compensation	197			274
Total deferred tax assets	34,102			40,300
Less: Valuation allowance	(34,102		)	(40,300		)
Net deferred tax assets	$	—		$	—

In assessing the realizability of deferred tax assets, management considers whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management believes it is more likely than not that the deferred tax assets will not be realized; accordingly, a valuation allowance has been established on U.S. net deferred tax assets. The valuation allowance decreased $3,832,000 during the year ended December 31, 2017 and increased by $6,197,000 during the year ended December 31, 2018.

As of December 31, 2018, the Company had net operating loss carryforwards of approximately $125,187,000 and $115,827,000 for federal and state income tax purposes, respectively. The federal and state net operating loss carryforwards begin to expire in 2027 and 2028, respectively.

The federal and state net operating loss carryforwards may be subject to significant limitations under Section 382 and Section 383 of the Internal Revenue Code and similar provisions under state law. The

Tax Reform Act contains provisions that limit the federal net operating loss carryforwards that may be used in any given year in the event of special occurrences, including significant ownership changes. A Section 382 “ownership change” generally occurs if one or more stockholders or groups of stockholders, who own at least 5% of the Company’s stock, increase their ownership by more than 50 percentage points over their lowest ownership percentage within a rolling three-year period. The Company may have previously experienced, and may in the future experience, one or more Section 382 “ownership changes,” including in connection with the Company’s initial public offering. If so, the Company may lose some or all of the tax benefits of its NOLs and tax credits. The extent of such limitations for prior years, if any, has not yet been determined.

At December 31, 2018, the Company had $4,083,000 and $2,655,000 of federal research and development tax credits and state tax credits, respectively. The state tax credits are made up of California Research and Development Credits and California New Jobs Credits. If not utilized, the Federal credits will expire beginning in 2027. The California Research and Development credits can be carried forward indefinitely, while the California New Jobs Credits begin to expire in 2019.

As of December 31, 2018, the Company had $1,348,000 of unrecognized tax benefits. The Company does not have any tax positions for which it is reasonably possible that the total amount of gross unrecognized would increase or decrease within twelve months of the year ended December 31, 2018. If recognized, $0 would affect the effective tax rate.

The Company recognizes interest and penalties related to uncertain tax positions in income tax expense. There was no such expense recorded during the years ended December 31, 2017 and 2018.

A reconciliation of the unrecognized tax benefits from January 1, 2017 to December 31, 2018 is as follows (in thousands):



	December 31,
	2017		2018
Balance at the beginning of year	$	551	$	615
Increases related to current years’ tax positions	64		118
Increases/(decreases) related to prior years’ tax positions	—		615
Balance at end of year	$	615	$	1,348

The Company currently has no federal or state tax examinations in progress nor has it had any federal or state tax examinations since its inception. As a result of the Company’s net operating loss carryforwards, all of its tax years are subject to federal and state tax examination.

On December 22, 2017, the United States enacted a law commonly known as the Tax Cuts and Jobs Act (“TCJA” or “Act”) which makes widespread changes to the Internal Revenue Code, including a reduction in the federal corporate tax rate to 21%, effective January 1, 2018. The Company is subject to the provisions of FASB ASC 740-10, which requires that the effect on deferred tax assets and liabilities of a change in tax rates be recognized in the period the tax rate change was enacted. Consequently, the reduction in the U.S. corporate income tax rate as a result of the TCJA impacts the carrying value of deferred tax assets.

In response to the Tax Act, the SEC staff issued guidance on accounting for the tax effects of the Tax Act. The guidance provided a one-year measurement period for companies to complete the accounting. During the year ended December 31, 2017, the Company completed its accounting for the income tax effects of the Tax Act.

Acquisition of Variable Interest Entity - NeuroCo	12 Months Ended
Acquisition of Variable Interest Entity - NeuroCo	Dec. 31, 2018
Business Combinations [Abstract]
Acquisition of Variable Interest Entity - NeuroCo	Acquisition of Variable Interest Entity - NeuroCo In December 2014, the Board of Directors of the Company approved the sale of certain intellectual property of Silk Road Medical, Inc., to a newly incorporated entity, NeuroCo, Inc. In consideration for the intellectual property, a promissory note was executed between the two parties for the principal sum of $498,000 with an interest rate of 2.74% per annum, payable on the earlier of 10 years from the date of promissory note, or upon the occurrence of an event of default. The intellectual property transfer was recorded at its carrying value of zero as of December 31, 2014. During 2015 NeuroCo issued $154,000 in common stock to stockholders of the Company. During the years ended December 31, 2017 and 2018, NeuroCo issued common stock upon the exercise of stock options. These common stock issuance amounts, as they are related to non-controlling investors, were reported as non-controlling interests in subsidiary in the Company’s consolidated financial statements and are offset by NeuroCo losses consolidated by the Company. Additionally, NeuroCo incurred Research and Development related expenses paid for by the Company which were added in to the original promissory note. As of December 31, 2017, the promissory note amount was $1,544,000. The Company had identified NeuroCo as a VIE of which the Company is the primary beneficiary. Pursuant to the accounting guidance for consolidating VIEs the main consideration was given to the fact that the amount of total equity investment at risk is not sufficient to permit NeuroCo to finance its activities without additional subordinated financial support. Additionally, NeuroCo and Silk Road Medical have the same Board of Directors and senior management composition, determining the Company to have the power to direct the activities that most significantly impact NeuroCo’s economic performance and the obligation to absorb losses and the right to receive benefits. Accordingly, the financial results of NeuroCo were included in the Company’s consolidated financial statements. On December 17, 2018, the Company and NeuroCo entered into the Agreement and Plan of Merger (the “Merger Agreement”) pursuant to which the Company acquired all assets and assumed all liabilities of NeuroCo (the “Merger”). The Merger closed on the same day (the “Closing”) and was consummated through a stock-for-stock transaction based on the relative values of the Company’s and NeuroCo’s equity. In consideration for 100% equity interest of NeuroCo, the Company issued 33,462 shares of its common stock and the above promissory note in the amount of approximately $1,600,000 as of the Closing was settled and canceled. As a result of the Merger, NeuroCo merged into the Company with the Company being the surviving corporation. As the Company already controlled and consolidated NeuroCo and retained the control over NeuroCo’s business after the Merger, the Company accounted for the acquisition of equity interest in NeuroCo as an equity transaction. Therefore, the Company did not recognize a gain or loss in its consolidated net loss or comprehensive loss for acquisition of NeuroCo. As the carrying amount of the non-controlling interest as of the Closing was zero, the Company recorded the consideration paid as a decrease to the Company’s additional paid-in capital within stockholder’s deficit. As part of the Merger, the Company assumed NeuroCo’s 2015 Equity Incentive Plan (the “NeuroCo Plan”) along with all of NeuroCo’s rights and obligations under the NeuroCo Plan, except that the number of shares and exercise price of the assumed options have been adjusted based on the Merger exchange ratio of the Company’s common stock and NeuroCo’s common stock. Similarly, the Company assumed outstanding warrants to purchase NeuroCo’s common stock such that the number of shares and exercise price of the assumed warrants have been adjusted based on the Merger exchange ratio of the Company’s common stock and NeuroCo’s common stock. The options and warrants to purchase shares of the Company’s common stock were fully vested upon issuance, as they were replacing fully vested options and warrants to purchase NeuroCo common stock.

401(k) Plan	12 Months Ended
401(k) Plan	Dec. 31, 2018
Retirement Benefits [Abstract]
401(k) Plan	401(k) Plan The Company has a qualified retirement plan under section 401(k) of the Internal Revenue Code (“IRC”) under which participants may contribute up to 90% of their eligible compensation, subject to maximum deferral limits specified by the IRC. The Company may make a discretionary matching contribution to the 401(k) plan and may make a discretionary employer contribution to each eligible employee each year. To date, the Company has made no contributions to the 401(k) plan.

Subsequent Events	3 Months Ended	12 Months Ended
Subsequent Events	Mar. 31, 2019	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Amended and Restated Certificate of Incorporation Effective upon its IPO, the Company filed an amended and restated certificate of incorporation that authorizes 100,000,000 shares of common stock, $0.001 par value per share, and 5,000,000 shares of preferred stock, $0.001 par value per share. 2019 Equity Incentive Plan The Company’s Board of Directors approved the grant of options to purchase 687,176 shares of common stock under the 2019 Equity Incentive Plan, with a grant date of April 3, 2019 with an exercise price equal to the initial public offering price of $20.00 per share. Long-term Debt Prior to the Company’s IPO, the term loans with CRG bore interest at a rate of 10.75%, which interest rate was further reduced to 10.00% upon the consummation of the Company’s IPO. At the Company’s election, it may pay the interest through a combination of cash and PIK. The interest is payable in cash and PIK as follows: 8.00% per annum in cash and 2.75% PIK; and post the consummation of the Company’s IPO, 8.00% per annum in cash and 2.00% PIK. Events Subsequent to Original Issuance of Condensed Consolidated Financial Statements (unaudited) In connection with the reissuance of the condensed consolidated financial statements, the Company has evaluated subsequent events through August 6, 2019, the date the condensed consolidated financial statements were available to be reissued. 2019 Equity Incentive Plan In May and June 2019, the Company’s Board of Directors approved the grant of options to purchase 36,982 shares of common stock under the 2019 Plan with a weighted average exercise price of $47.54 per share. Long-term Debt In June 2019, the Company entered into Amendment No. 7 to the term loan agreement with CRG to reflect flexibility with respect to permitted cash equivalents. Legal Matters The Company and the former employee participated in mediation on July 30, 2019 and reached a tentative settlement that would require the Company to pay an amount that is not material to its consolidated financial statements.	Subsequent Events The Company has evaluated subsequent events through March 1, 2019, which is the date these audited consolidated financial statements were available for issuance. The Company has also evaluated subsequent events through March 27, 2019 for the effects of the reverse stock split described in Note 1. Contingencies In February 2019, a former employee, through counsel, advised the Company that he had filed a charge of discrimination against the Company with the California Department of Fair Employment & Housing, or DFEH. The former employee’s complaint alleges sexual harassment and retaliation in violation of the California Department of Fair Employment & Housing Act. The complaint does not allege specific damages. To date, the DFEH has not contacted the Company. The Company denies the complaint’s allegations and intends to vigorously defend itself. At this time the Company cannot estimate the outcomes or possible loss or range of loss arising from this claim, if any; as such, no accrual was included in the Company’s balance sheet as of December 31, 2018. 2007 Stock Option Plan In February 2019, the Company’s Board of Directors approved the grant of options to purchase 32,950 shares of common stock under the 2007 Stock Option Plan at an exercise price of $11.29 per share. These stock options have a grant date fair value of approximately $160,000 that is expected to be recognized over a requisite service period of four years.

Events Subsequent to Original Issuance of Condensed Consolidated Financial Statements (unaudited)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Subsequent Events [Abstract]
Events Subsequent to Original Issuance of Condensed Consolidated Financial Statements (unaudited)	Subsequent Events Amended and Restated Certificate of Incorporation Effective upon its IPO, the Company filed an amended and restated certificate of incorporation that authorizes 100,000,000 shares of common stock, $0.001 par value per share, and 5,000,000 shares of preferred stock, $0.001 par value per share. 2019 Equity Incentive Plan The Company’s Board of Directors approved the grant of options to purchase 687,176 shares of common stock under the 2019 Equity Incentive Plan, with a grant date of April 3, 2019 with an exercise price equal to the initial public offering price of $20.00 per share. Long-term Debt Prior to the Company’s IPO, the term loans with CRG bore interest at a rate of 10.75%, which interest rate was further reduced to 10.00% upon the consummation of the Company’s IPO. At the Company’s election, it may pay the interest through a combination of cash and PIK. The interest is payable in cash and PIK as follows: 8.00% per annum in cash and 2.75% PIK; and post the consummation of the Company’s IPO, 8.00% per annum in cash and 2.00% PIK. Events Subsequent to Original Issuance of Condensed Consolidated Financial Statements (unaudited) In connection with the reissuance of the condensed consolidated financial statements, the Company has evaluated subsequent events through August 6, 2019, the date the condensed consolidated financial statements were available to be reissued. 2019 Equity Incentive Plan In May and June 2019, the Company’s Board of Directors approved the grant of options to purchase 36,982 shares of common stock under the 2019 Plan with a weighted average exercise price of $47.54 per share. Long-term Debt In June 2019, the Company entered into Amendment No. 7 to the term loan agreement with CRG to reflect flexibility with respect to permitted cash equivalents. Legal Matters The Company and the former employee participated in mediation on July 30, 2019 and reached a tentative settlement that would require the Company to pay an amount that is not material to its consolidated financial statements.	Subsequent Events The Company has evaluated subsequent events through March 1, 2019, which is the date these audited consolidated financial statements were available for issuance. The Company has also evaluated subsequent events through March 27, 2019 for the effects of the reverse stock split described in Note 1. Contingencies In February 2019, a former employee, through counsel, advised the Company that he had filed a charge of discrimination against the Company with the California Department of Fair Employment & Housing, or DFEH. The former employee’s complaint alleges sexual harassment and retaliation in violation of the California Department of Fair Employment & Housing Act. The complaint does not allege specific damages. To date, the DFEH has not contacted the Company. The Company denies the complaint’s allegations and intends to vigorously defend itself. At this time the Company cannot estimate the outcomes or possible loss or range of loss arising from this claim, if any; as such, no accrual was included in the Company’s balance sheet as of December 31, 2018. 2007 Stock Option Plan In February 2019, the Company’s Board of Directors approved the grant of options to purchase 32,950 shares of common stock under the 2007 Stock Option Plan at an exercise price of $11.29 per share. These stock options have a grant date fair value of approximately $160,000 that is expected to be recognized over a requisite service period of four years.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Preparation

Principles of Consolidation

Variable Interest Entity

Use of Estimates

Fair Value of Financial Instruments

approximates its fair value. Fair value accounting is applied to the redeemable convertible preferred stock warrant liability.

Fair Value of Financial Instruments

Cash, Cash Equivalents and Restricted Cash

The Company considers all highly liquid investments with an original maturity of three months or less at the time of purchase to be cash equivalents. Cash equivalents are considered available-for-sale marketable securities and are recorded at fair value, based on quoted market prices. As of December 31, 2017 and 2018, the Company’s cash equivalents are entirely comprised of investments in money market funds.



	December 31,
	2017		2018
Cash and cash equivalents	$	33,331	$	24,990
Restricted cash	510		310
Total cash, cash equivalents and restricted cash	$	33,841	$	25,300

Concentration of Credit Risk, and Other Risks and Uncertainties

Deferred Initial Public Offering Costs

Specific incremental legal, accounting and other fees and costs directly attributable to a proposed or actual offering of securities may properly be deferred and charged against the gross proceeds of the offering. In the event the Company’s planned IPO does not occur or is significantly delayed, all of the costs will be expensed. As of December 31, 2018 there were $950,000 of offering costs primarily consisting of legal and accounting fees that were capitalized in other non-current assets on the consolidated balance sheet. No deferred offering costs were capitalized as of December 31, 2017.

Leases

In accordance with ASC 842, the disclosure impact of adoption on the condensed consolidated balance sheet were as follows (in thousands):



Balance Sheet:	Balance at December 31, 2018		Adjustments Due to ASC 842		Balance at January 1, 2019
Other non-current assets	$	—	$	3,982	$	3,982
Accrued liabilities	139		582		721
Other liabilities	1,069		3,400		4,469

separately for contracts that contain lease and non-lease components consistent with its historical practice. Variable lease payments will be expensed as incurred.

Redeemable Convertible Preferred Stock Warrant Liability

The Company accounts for its warrants for shares of redeemable convertible preferred stock as a liability based upon the characteristics and provisions of each instrument. Redeemable convertible preferred stock warrants classified as a liability are initially recorded at their fair value on the date of issuance and are subject to remeasurement at each subsequent balance sheet date. Any change in fair value as a result of a remeasurement is recognized as a component of other income (expense), net in the consolidated statements of operations and comprehensive loss.

Redeemable Convertible Preferred Stock

Revenue Recognition and Cost of Goods Sold

Revenue Recognition


(i)	identify the contract(s) with a customer;


(ii)	identify the performance obligations in the contract;


(iii)	determine the transaction price;


(iv)	allocate the transaction price to the performance obligations in the contract; and


(v)	recognize revenue when (or as) the entity satisfies a performance obligation.

Under ASC 606, assuming all other revenue recognition criteria have been met, the Company will recognize revenue earlier for arrangements where the Company has satisfied its performance obligations

Cost of Goods Sold

Revenue Recognition


(i)	identify the contract(s) with a customer;


(ii)	identify the performance obligations in the contract;


(iii)	determine the transaction price;


(iv)	allocate the transaction price to the performance obligations in the contract; and


(v)	recognize revenue when (or as) the entity satisfies a performance obligation.



	Balance at December 31, 2017			Adjustments Due to ASC 606			Balance at January 1, 2018
Accounts receivable, net	$	5,215		$	136		$	5,351
Inventories	3,248			(46		)	3,202
Accrued liabilities	3,109			4			3,113
Accumulated deficit	(101,556		)	87			(101,469		)

In accordance with ASC 606, the disclosure of the impact of adoption on the consolidated balance sheet and statement of operations and comprehensive loss were as follows (in thousands):



	Year Ended December 31, 2018
	Balance As Reported			Balance Before ASC 606 Adoption			Effect of Change
Balance sheet:
Accounts receivable, net	$	4,520		$	4,494		$	26
Inventories	5,744			5,766			(22		)
Accrued liabilities	7,586			7,586			—
Accumulated deficit	(139,111		)	(139,107		)	(4		)
Statement of operations and comprehensive loss:
Revenue	34,557			34,583			(26		)
Cost of goods sold	10,874			10,852			22

Costs associated with product sales include commissions and royalties. The Company applies the practical expedient and recognizes commissions and royalties as expense when incurred because the expense is incurred at a point in time and the amortization period is less than one year. Commissions are recorded as selling expense and royalties are recorded as cost of revenue in the consolidated statements of operations and comprehensive loss.


•	persuasive evidence of an arrangement exists;


•	the sales price is fixed or determinable;


•	collection of the relevant receivable is reasonably assured at the time of sale; and


•	delivery has occurred or services have been rendered.

Cost of Goods Sold

Research and Development and Clinical Trials

Research and Development

Clinical Trials

Advertising Costs

Foreign Currency

Stock-Based Compensation

Stock–Based Compensation

Income Taxes

The Company also follows the provisions of ASC 740-10, “Accounting for Uncertainty in Income Taxes.” ASC 740-10 prescribes a comprehensive model for the recognition, measurement, presentation and disclosure in financial statements of any uncertain tax positions that have been taken or expected to be taken on a tax return. No liability related to uncertain tax positions is recorded on the consolidated financial statements. It is the Company’s policy to include penalties and interest expense related to income taxes as part of the provision for income taxes.

Net Loss per Share Attributable to Common Stockholders and Unaudited Pro Forma Net Loss per Share Attributable to Common Stockholders

Net Loss per Share Attributable to Common Stockholders

Net loss per share was determined as follows (in thousands, except share and per share data):



	Three Months Ended March 31,
	2018			2019
Net loss attributable to Silk Road Medical, Inc. common stockholders	$	(5,408	)	$	(24,158	)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	739,308			1,200,719
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted	$	(7.31	)	$	(20.12	)



	March 31,
	2018	2019
Redeemable convertible preferred stock outstanding	21,233,190	21,238,105
Redeemable convertible preferred stock warrants outstanding	2,672,502	2,667,587
Common stock options	4,431,797	4,138,635
Common stock warrants outstanding	—	7,527
	28,337,489	28,051,854

Pro Forma Net Loss per Share Attributable to Common Stockholders

Unaudited pro forma basic and diluted loss per share is computed as follows (in thousands, except share and per share data):



	Three Months Ended March 31,
	2019

Numerator:
Net loss and comprehensive loss attributable to Silk Road Medical, Inc. common stockholders	$	(24,158	)
Adjust: Change in fair value of redeemable convertible preferred stock warrants	15,712
Pro forma net loss	$	(8,446	)
Denominator:
Weighted average common shares used to compute net loss per share, basic and diluted	1,200,719
Adjust: Conversion of redeemable convertible preferred stock	21,233,190
Adjust: Cash and net exercise of redeemable convertible preferred stock warrants into common stock	1,941,105
Adjust: Cash and net exercise of common stock warrants into common stock	5,970
Weighted average common shares used to compute pro forma net loss per share, basic and diluted	24,380,984
Pro forma net loss per share, basic and diluted	$	(0.35	)

Net Loss per Share Attributable to Common Stockholders

Net loss per share was determined as follows (in thousands, except share and per share data):



	Year Ended December 31,
	2017			2018
Net loss attributable to Silk Road Medical, Inc. common stockholders	$	(19,356	)	$	(37,629	)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	434,158			960,882
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted	$	(44.58	)	$	(39.16	)



	December 31,
	2017	2018
Redeemable convertible preferred stock outstanding	21,233,190	21,233,190
Redeemable convertible preferred stock warrants outstanding	2,672,502	2,672,502
Common stock options	4,308,890	4,364,377
Common stock warrants outstanding	—	7,527
	28,214,582	28,277,596

Unaudited Pro Forma Net Loss per Share Attributable to Common Stockholders

Comprehensive Loss

For the three months ended March 31, 2018 and March 31, 2019, there was no difference between comprehensive loss and the Company’s net loss.

Comprehensive Loss

For the years ended December 31, 2017 and 2018, there was no difference between comprehensive loss and the Company’s net loss.

Segment and Geographical Information

The Company operates and manages its business as one reportable and operating segment. The Company’s chief executive officer, who is the chief operating decision maker, reviews financial information on an aggregate basis for purposes of allocating resources and evaluating financial performance. All of the Company’s long-lived assets are based in the United States. Long-lived assets are comprised of property and equipment. All of the Company’s revenue was in the United States for the years ended December 31, 2017 and 2018, based on the shipping location of the external customer.

Recent Accounting Pronouncements

Recently Adopted Accounting Standards

In the first quarter of 2018, the Company adopted ASU No. 2014-09, Revenue from Contracts with Customers (Topic 606), and its associated amendments. Under the standard, revenue is recognized when a customer obtains control of promised goods or services in an amount that reflects the consideration the entity expects to receive in exchange for those goods or services. In addition, the standard requires disclosure of the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The Company applied the five step method outlined in the ASU to all revenue streams and elected to utilize the modified retrospective implementation method. The additional disclosures required by the ASU have been included in Note 2, “Summary of Significant Accounting Policies.”

In the first quarter of 2018, the Company adopted ASU No. 2016-18, Statement of Cash Flows: Restricted Cash, a consensus of the Financial Accounting Standards Board (“FASB”) Emerging Issues Task Force. Under the standard, restricted cash and restricted cash equivalent amounts are presented within cash and cash equivalents when reconciling the total beginning and ending amounts shown on the statement of cash flows. When cash, cash equivalents, restricted cash and restricted cash equivalents are presented in more than one line item on the balance sheet, a reconciliation of the totals in the statement of cash flows to the related captions in the balance sheet is required. The impact of the adoption of ASU No. 2016-18 resulted in a decrease in investing activities of $310,000 and an increase in the ending cash, cash equivalents and restricted cash of $510,000 in the consolidated statement of cash flows for the year ended December 31, 2017. The impact of the adoption resulted in a decrease in investing activities and an increase in the ending cash, cash equivalents and restricted cash of $200,000 in the consolidated statement of cash flows for the year ended December 31, 2018.

In the first quarter of 2018, the Company adopted ASU No. 2017-09, Compensation - Stock Compensation - Scope of Modification Accounting. The standard provides clarification on when modification accounting should be used for changes to the terms or conditions of a share-based payment award. This standard does not change the accounting for modifications but clarifies that modification accounting guidance should only be applied if there is a change to the value, vesting conditions, or award classification and would not be required if the changes are considered non-substantive. The guidance was adopted on a prospective basis in the first quarter of 2018 and did not have any impact upon adoption.

In the first quarter of 2018, the Company adopted ASU No. 2018-07, Improvements to Nonemployee Share-Based Payment Accounting. ASU 2018-07 simplifies the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. Adoption of this guidance did not have a material impact on the Company’s consolidated financial statements.

Recently Issued Accounting Standards

In February 2016, the FASB issued ASU 2016-02, Leases (“ASC 842”), which sets out the principles for the recognition, measurement, presentation and disclosure of leases for both parties to a contract (i.e. lessees and lessors). The new standard requires lessees to apply a dual approach, classifying leases as either finance or operating leases based on the principle of whether or not the lease is effectively a financed purchase by the lessee. This classification will determine whether lease expense is recognized based on an effective interest method or on a straight-line basis over the term of the lease, respectively. A lessee is also required to record a right-of-use asset and a lease liability for all leases with a term of greater than 12 months regardless of their classification. Leases with a term of 12 months or less will be accounted for similar to existing guidance for operating leases today. ASC 842 supersedes the previous

leases standard, ASC 840, Leases. For public entities, the standard is effective for interim and annual periods beginning after December 15, 2018 and should be applied through a modified retrospective transition approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with early adoption permitted. The Company plans to adopt the new standard on January 1, 2019 and elect the optional transition method. The Company will also elect the package of transitional practical expedients such that the Company will retain lease classification and initial direct costs for leases existing prior to the adoption of the new lease standard. The Company will also elect the hindsight practical expedient. Although the Company is currently evaluating the impact of this guidance on its consolidated financial statements, it does expect that most of its operating lease commitments will be subject to the new guidance and will be recognized as operating lease liabilities and right-of-use assets upon its adoption.

In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Statements. This update provides financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The update replaces the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU No. 2016-13 is effective for public entities for annual periods beginning after December 15, 2019. The Company does not believe that the adoption of this new guidance will have a material impact on its consolidated financial statements.

In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement, which changed the disclosure requirements for fair value measurements by removing, adding and modifying certain disclosures. The standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU 2018-15, Cloud Computing Arrangements, which aligns the requirements for capitalizing implementation costs in a Cloud Computing Arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. The standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures.

Recent Accounting Pronouncements

Recently Adopted Accounting Standards

In February 2016, the FASB issued ASU No. 2016-02, “Leases,” that supersedes Accounting Standards Codification (“ASC”) 840, “Leases.” Subsequently, the FASB issued several updates to ASU No. 2016-02, codified in ASC Topic 842 (“ASC 842”). The Company adopted ASC 842 on January 1, 2019 using the modified retrospective method for all leases not substantially completed as of the date of adoption. The Company elected to apply the package of practical expedients, which allowed the Company to not reassess: (i) whether expired or existing contracts contain leases; (ii) lease classification for any expired or existing leases; and (iii) initial direct costs for any existing lease.

Recently Issued Accounting Standards

In June 2016, the FASB issued Accounting Standards Update (“ASU”) 2016-13, “Measurement of Credit Losses on Financial Statements.” This update provides financial statement users with more decision-useful information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. The update replaces the incurred loss impairment methodology in current GAAP with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU No. 2016-13 is effective for public entities for annual periods beginning after December 15, 2019. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU 2018-13, “Fair Value Measurement,” which changed the disclosure requirements for fair value measurements by removing, adding and modifying certain disclosures. The standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures.

In August 2018, the FASB issued ASU 2018-15, “Cloud Computing Arrangements,” which aligns the requirements for capitalizing implementation costs in a Cloud Computing Arrangement service contract with the requirements for capitalizing implementation costs incurred for an internal-use software license. The standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted. The Company is evaluating the impact of adopting this guidance to its consolidated financial statements and related disclosures.

Summary of Significant Accounting Policies (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Accounting Policies [Abstract]

Restrictions on Cash and Cash Equivalents



	December 31,
	2017		2018
Cash and cash equivalents	$	33,331	$	24,990
Restricted cash	510		310
Total cash, cash equivalents and restricted cash	$	33,841	$	25,300

Schedule of Cash and Cash Equivalents



	December 31,
	2017		2018
Cash and cash equivalents	$	33,331	$	24,990
Restricted cash	510		310
Total cash, cash equivalents and restricted cash	$	33,841	$	25,300

Schedule of New Accounting Pronouncements and Changes in Accounting Principles

The cumulative effect of the changes made to the consolidated balance sheet as of January 1, 2018 for the adoption of ASC 606 were as follows (in thousands):



	Balance at December 31, 2017			Adjustments Due to ASC 606			Balance at January 1, 2018
Accounts receivable, net	$	5,215		$	136		$	5,351
Inventories	3,248			(46		)	3,202
Accrued liabilities	3,109			4			3,113
Accumulated deficit	(101,556		)	87			(101,469		)

In accordance with ASC 606, the disclosure of the impact of adoption on the consolidated balance sheet and statement of operations and comprehensive loss were as follows (in thousands):



	Year Ended December 31, 2018
	Balance As Reported			Balance Before ASC 606 Adoption			Effect of Change
Balance sheet:
Accounts receivable, net	$	4,520		$	4,494		$	26
Inventories	5,744			5,766			(22		)
Accrued liabilities	7,586			7,586			—
Accumulated deficit	(139,111		)	(139,107		)	(4		)
Statement of operations and comprehensive loss:
Revenue	34,557			34,583			(26		)
Cost of goods sold	10,874			10,852			22

Schedule of ASC 842 Impact of Adoption on Condensed Consolidated Balance Sheet

In accordance with ASC 842, the disclosure impact of adoption on the condensed consolidated balance sheet were as follows (in thousands):



Balance Sheet:	Balance at December 31, 2018		Adjustments Due to ASC 842		Balance at January 1, 2019
Other non-current assets	$	—	$	3,982	$	3,982
Accrued liabilities	139		582		721
Other liabilities	1,069		3,400		4,469

Balance sheet information as of March 31, 2019 consists of the following (in thousands):



Operating Lease:	March 31, 2019
Operating lease right-of-use asset in other non-current assets	$	3,820
Operating lease liability in accrued liabilities	$	712
Operating lease liability in other liabilities	4,285
Total operating lease liabilities	$	4,997

Net Loss Per Share Determination

Net loss per share was determined as follows (in thousands, except share and per share data):



	Three Months Ended March 31,
	2018			2019
Net loss attributable to Silk Road Medical, Inc. common stockholders	$	(5,408	)	$	(24,158	)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	739,308			1,200,719
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted	$	(7.31	)	$	(20.12	)

Net loss per share was determined as follows (in thousands, except share and per share data):



	Year Ended December 31,
	2017			2018
Net loss attributable to Silk Road Medical, Inc. common stockholders	$	(19,356	)	$	(37,629	)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted	434,158			960,882
Net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted	$	(44.58	)	$	(39.16	)

Schedule of Potentially Dilutive Securities Outstanding Excluded from Diluted Weighted Average Shares Outstanding



	March 31,
	2018	2019
Redeemable convertible preferred stock outstanding	21,233,190	21,238,105
Redeemable convertible preferred stock warrants outstanding	2,672,502	2,667,587
Common stock options	4,431,797	4,138,635
Common stock warrants outstanding	—	7,527
	28,337,489	28,051,854



	December 31,
	2017	2018
Redeemable convertible preferred stock outstanding	21,233,190	21,233,190
Redeemable convertible preferred stock warrants outstanding	2,672,502	2,672,502
Common stock options	4,308,890	4,364,377
Common stock warrants outstanding	—	7,527
	28,214,582	28,277,596

Schedule of Calculation of Pro Forma Earnings Per Share

Unaudited pro forma basic and diluted loss per share is computed as follows (in thousands, except share and per share data):



	Three Months Ended March 31,
	2019

Numerator:
Net loss and comprehensive loss attributable to Silk Road Medical, Inc. common stockholders	$	(24,158	)
Adjust: Change in fair value of redeemable convertible preferred stock warrants	15,712
Pro forma net loss	$	(8,446	)
Denominator:
Weighted average common shares used to compute net loss per share, basic and diluted	1,200,719
Adjust: Conversion of redeemable convertible preferred stock	21,233,190
Adjust: Cash and net exercise of redeemable convertible preferred stock warrants into common stock	1,941,105
Adjust: Cash and net exercise of common stock warrants into common stock	5,970
Weighted average common shares used to compute pro forma net loss per share, basic and diluted	24,380,984
Pro forma net loss per share, basic and diluted	$	(0.35	)

Unaudited pro forma basic and diluted loss per share is computed as follows (in thousands, except share and per share data):



	Year Ended December 31,
	2018
	(unaudited)
Numerator:
Net loss and comprehensive loss attributable to Silk Road Medical, Inc. common stockholders	$	(37,629	)
Adjust: Change in fair value of redeemable convertible preferred stock warrants	11,906
Pro forma net loss	$	(25,723	)
Denominator:
Weighted average common shares used to compute net loss per share, basic and diluted	960,882
Adjust: Conversion of redeemable convertible preferred stock	21,233,190
Adjust: Cash and net exercise of redeemable convertible preferred stock warrants into common stock warrants	1,940,450
Adjust: Cash and net exercise of common stock warrants into common stock	246
Weighted average common shares used to compute pro forma net loss per share, basic and diluted	24,134,768
Pro forma net loss per share, basic and diluted	$	(1.07	)

Fair Value Measurements (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Option Pricing Model Assumptions

Additionally, the model’s inputs reflect assumptions that market participants would use in pricing the instrument in a current period transaction and included:



	December 31,
	2018
Time to liquidity (years)	0.57
Expected volatility	62.5%
Discounted cash flow rate	12.0%
Risk-free interest rate	2.6%
Marketability discount rate	14%

Additionally, the model’s inputs reflect assumptions that market participants would use in pricing the instrument in a current period transaction and included:



	Year Ended December 31,
	2017	2018
Time to liquidity (years)	1.75	0.57
Expected volatility	50.0%	62.5%
Discounted cash flow rate	18.0%	12.0%
Risk-free interest rate	1.9%	2.6%
Marketability discount rate	27%	14%

Financial Liabilities Measure on a Recurring Basis

The following table sets forth the fair value of the Company’s financial liabilities measured on a recurring basis, as of December 31, 2018 and March 31, 2019 (in thousands):



	December 31, 2018
	Level 1		Level 2		Level 3		Total
Liabilities
Redeemable convertible warrant liability	$	—	$	—	$	16,091	$	16,091



	March 31, 2019
	Level 1		Level 2		Level 3		Total
Liabilities
Redeemable convertible warrant liability	$	—	$	—	$	31,803	$	31,803

The following table sets forth the fair value of the Company’s financial liabilities measured on a recurring basis, as of December 31, 2017 and 2018 (in thousands):



	December 31, 2017
	Level 1		Level 2		Level 3		Total
Liabilities
Redeemable convertible warrant liability	$	—	$	—	$	4,185	$	4,185



	December 31, 2018
	Level 1		Level 2		Level 3		Total
Liabilities
Redeemable convertible warrant liability	$	—	$	—	$	16,091	$	16,091

Changes in the Redeemable Convertible Warrant Liability

The changes in the redeemable convertible warrant liability are summarized below (in thousands):



Fair Value at December 31, 2018	$	16,091
Change in fair value recorded in other income (expense), net	15,712
Fair Value at March 31, 2019	$	31,803

The changes in the redeemable convertible warrant liability are summarized below (in thousands):



Fair value at December 31, 2016	$	7,143
Change in fair value recorded in other income (expense), net	(2,958		)
Fair value at December 31, 2017	4,185
Change in fair value recorded in other income (expense), net	11,906
Fair value at December 31, 2018	$	16,091

Balance Sheet Components (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of Inventories

Inventories



(in thousands)	December 31,		March 31,
	2018		2019
Raw materials	$	1,054	$	2,147
Finished products	4,690		5,909
	$	5,744	$	8,056

Inventories



(in thousands)	December 31,
	2017		2018
Raw materials	$	506	$	1,054
Finished products	2,742		4,690
	$	3,248	$	5,744

Property, Plant and Equipment

Property and Equipment, Net



(in thousands)	December 31,
	2017			2018
Furniture and fixtures	$	76		$	517
Equipment	1,059			1,217
Software	405			76
Leasehold improvements	189			1,978
	1,729			3,788
Less: Accumulated depreciation and amortization	(1,303		)	(946		)
Add: Construction-in-progress	60			38
	$	486		$	2,880

Schedule of Accrued Liabilities

Accrued Liabilities



(in thousands)	December 31,
	2017		2018
Accrued payroll and related expenses	$	2,718	$	5,157
Accrued professional services	64		1,014
Accrued royalty expense	—		313
Accrued travel expenses	68		270
Accrued clinical expenses	183		244
Accrued other expenses	76		588
	$	3,109	$	7,586

Long-term Debt (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Debt Disclosure [Abstract]

Future Maturities Under the Term Loan Agreement

Future maturities under the term loan agreement as of March 31, 2019 are as follows (in thousands):



Period Ending December 31:	Amount
2019	$	2,696
2020	3,677
2021	3,771
2022	52,510
	62,654
Add: Accretion of closing fees	958
	63,612
Less: Amount representing interest	(18,841		)
Less: Amount representing debt discount and debt issuance costs	(174		)
Present value of minimum payments	$	44,597

Future maturities under the term loan agreement as of December 31, 2018 are as follows (in thousands):



Year Ending December 31:	Amount
2019	$	3,566
2020	3,677
2021	3,771
2022	52,510
	63,524
Add: Accretion of closing fees	872
	64,396
Less: Amount representing interest	(20,010		)
Less: Amount representing debt discount and debt issuance costs	(185		)
Present value of minimum payments	$	44,201

Commitments and Contingencies (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Balance Sheet Information

In accordance with ASC 842, the disclosure impact of adoption on the condensed consolidated balance sheet were as follows (in thousands):



Balance Sheet:	Balance at December 31, 2018		Adjustments Due to ASC 842		Balance at January 1, 2019
Other non-current assets	$	—	$	3,982	$	3,982
Accrued liabilities	139		582		721
Other liabilities	1,069		3,400		4,469

Balance sheet information as of March 31, 2019 consists of the following (in thousands):



Operating Lease:	March 31, 2019
Operating lease right-of-use asset in other non-current assets	$	3,820
Operating lease liability in accrued liabilities	$	712
Operating lease liability in other liabilities	4,285
Total operating lease liabilities	$	4,997

Operating Lease Maturities

The following table summarizes the Company’s operating lease maturities as of March 31, 2019 (in thousands):



Period Ending December 31:	Amount
2019	$	760
2020	1,037
2021	1,066
2022	1,096
2023	1,127
2024	904
Total lease payments	5,990
Less: imputed interest	(993		)
Present value of lease liabilities	$	4,997

Minimum Future Lease Payment Previously Disclosed Under ASC 840



Year Ending December 31:	Total Minimum Lease Payments
2019	$	1,002
2020	1,002
2021	1,031
2022	1,044
2023	1,920
	$	5,999

The aggregate future minimum lease payments as of December 31, 2018 are as follows (in thousands):



Year Ending December 31:	Total Minimum Lease Payments
2019	$	1,002
2020	1,002
2021	1,031
2022	1,044
2023 and thereafter	1,920
	$	5,999

Redeemable Convertible Preferred Stock (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Redeemable Convertible Preferred Stock Issued and Outstanding

The Company has the following redeemable convertible preferred stock issued and outstanding at December 31, 2018:



	December 31, 2018
	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,227,992	$	6.11	74,713		59,129
	24,069,615	21,233,190			$	121,144	$	105,235

The Company has the following redeemable convertible preferred stock issued and outstanding at March 31, 2019:



	March 31, 2019
	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,232,907	$	6.11	74,743		59,159
	24,069,615	21,238,105			$	121,174	$	105,265

The Company has the following redeemable convertible preferred stock issued and outstanding at December 31, 2017 and 2018:



	December 31, 2017 and 2018
	Shares Authorized	Shares Issued and Outstanding	Per share Preference		Preferential Liquidation Value (in thousands)		Carrying Value (in thousands)
Series
Series A	1,629,629	1,629,626	$	2.70	$	4,400	$	4,369
Series A-1	1,111,111	1,111,109	$	3.38	3,755		3,723
Series B	6,264,470	6,264,463	$	6.11	38,276		38,014
Series C	15,064,405	12,227,992	$	6.11	74,713		59,129
	24,069,615	21,233,190			$	121,144	$	105,235

Common Stock (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Equity [Abstract]

Reserved Common Stock for Future Issuances

At December 31, 2018 and March 31, 2019, the Company had reserved common stock for future issuances as follows:



	December 31,	March 31,
	2018	2019
Conversion of Series A convertible preferred stock	1,629,629	1,629,629
Conversion of Series A-1 convertible preferred stock	1,111,111	1,111,111
Conversion of Series B convertible preferred stock	6,264,470	6,264,470
Conversion of Series C convertible preferred stock and warrants	15,064,405	15,064,405
Exercise of options under stock plan	4,364,377	4,138,635
Issuance of options under stock plan	57,889	32,326
Warrants to purchase common stock	7,527	7,527
	28,499,408	28,248,103

At December 31, 2017 and 2018, the Company had reserved common stock for future issuances as follows:



	December 31,
	2017	2018
Conversion of Series A convertible preferred stock	1,629,629	1,629,629
Conversion of Series A-1 convertible preferred stock	1,111,111	1,111,111
Conversion of Series B convertible preferred stock	6,264,470	6,264,470
Conversion of Series C convertible preferred stock and warrants	15,064,405	15,064,405
Exercise of options under stock plan	4,308,890	4,364,377
Issuance of options under stock plan	328,290	57,889
Warrants to purchase common stock	—	7,527
	28,706,795	28,499,408

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

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Reference 7: http://fasb.org/us-gaap/role/ref/legacyRef
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-Subparagraph (SX 210.4-08(d))
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+ Details

Stock Option Plan (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2019

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Share-based Compensation, Stock Options, Activity

Activity under the Company’s Plan and the NeuroCo Plan is set forth below:



			Options Outstanding
	Shares Available for Grant		Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value (in thousands)
Balances, December 31, 2016	27,605		3,478,346		$	1.46	7.71	$	444
Authorized	1,421,867
Options granted	(1,375,329	)	1,375,329		$	6.54
Options exercised	—		(290,638	)	$	1.17
Options cancelled	254,147		(254,147	)	$	1.55
Balances, December 31, 2017	328,290		4,308,890		$	3.09	7.81	$	5,073
Authorized	223,664
Options granted	(629,716	)	629,716		$	6.51
Options exercised	—		(438,578	)	$	1.50
Options cancelled	135,651		(135,651	)	$	1.56
Balances December 31, 2018	57,889		4,364,377		$	3.79	7.36	$	33,132
Vested and exercisable at December 31, 2018			2,459,116		$	2.35	6.28	$	22,109
Vested and expected to vest at December 31, 2018			4,464,377		$	3.79	7.36	$	33,132

Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price

The following table summarizes information about stock options outstanding at December 31, 2018:



	Options Outstanding and Vested as of December 31, 2018
	Options Outstanding				Options Vested
Exercise Price	Options Outstanding	Weighted Average Remaining Contractual Term (in Years)	Weighted Average Exercise Price		Number Exercisable	Weighted Average Exercise Price
$0.68	101,848	1.59	$	0.68	101,848	$	0.68
$1.35	107,938	2.75	$	1.35	107,938	$	1.35
$1.38	304,396	3.96	$	1.38	304,396	$	1.38
$1.46	385,070	6.21	$	1.46	360,470	$	1.46
$1.57	97,311	8.07	$	1.57	42,013	$	1.57
$1.60	1,463,632	6.82	$	1.60	1,128,569	$	1.60
$3.16	462,764	8.95	$	3.16	162,549	$	3.16
$4.73	332,385	8.92	$	4.73	86,506	$	4.73
$6.11	552,462	9.32	$	6.11	29,848	$	6.11
$8.27	78,941	9.93	$	8.27	1,527	$	8.27
$12.15	477,630	8.92	$	12.15	133,452	$	12.15
	4,364,377	7.36	$	3.79	2,459,116	$	2.35

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions

The fair value of employee stock options was estimated using the following assumptions for the years ended December 31, 2017 and 2018:



	Year Ended December 31,
	2017	2018
Expected term (in years)	1.00 - 6.25	6.25
Expected volatility	39% - 41%	38.1% - 38.8%
Risk-free interest rate	1.03% - 2.25%	2.68% - 2.98%
Dividend yield	—%	—%

The fair value of stock options granted to nonemployees was calculated using the following assumptions:



	Year Ended December 31,
	2017	2018
Contractual term (in years)	3.75 - 9.75	5.00 - 6.25
Expected volatility	39% - 56%	38.0% - 38.8%
Risk-free interest rate	2.06% - 2.39%	2.71% - 2.90%
Dividend yield	—%	—%

Stock-Based Compensation Expense Relating to Stock Options to Employees and Nonemployees

Total stock-based compensation expense relating to the Company’s stock options to employees and nonemployees during the three months ended March 31, 2018 and 2019, is as follows (in thousands):



	Three Months Ended March 31,
	2018		2019
Cost of goods sold	$	13	$	15
Research and development expenses	29		27
Selling, general and administrative expenses	120		220
	$	162	$	262

Total stock-based compensation expense relating to the Company’s stock options to employees and nonemployees during the years ended December 31, 2017 and 2018, is as follows (in thousands):



	Year Ended December 31,
	2017		2018
Cost of goods sold	$	49	$	51
Research and development expenses	98		256
Selling, general and administrative expenses	388		604
	$	535	$	911

Disclosure of Share-based Compensation Arrangements by Share-based Payment Award

The fair value of employee stock options was estimated using the following assumptions for the three months ended March 31, 2018 and 2019:



	Three Months Ended March 31,
	2018	2019
Expected term (in years)	6.25	6.25
Expected volatility	38.1%	42.7%
Risk-free interest rate	2.68%	2.54%
Dividend yield	—%	—%

Activity under the Company’s Plan is set forth below:



			Options Outstanding
	Shares Available for Grant		Number of Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in Years)	Aggregate Intrinsic Value (in thousands)
Balances, December 31, 2018	57,889		4,364,377		$	3.79	7.36	$	33,132
Authorized	—
Options granted	(32,950	)	32,950		$	11.29
Options exercised	—		(251,305	)	$	1.50
Options cancelled	7,387		(7,387	)	$	1.56
Balances, March 31, 2019	32,326		4,138,635		$	4.00	7.26	$	30,602
Vested and exercisable at March 31, 2019			2,451,044		$	2.70	6.36	$	21,195
Vested and expected to vest at March 31, 2019			4,138,635		$	4.00	7.26	$	30,602

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Components of Income Tax Expense (Benefit)

The components of income before taxes are as follows (in thousands):



	Year Ended December 31,
	2017			2018
United States	$	(22,358	)	$	(37,630	)
International	—			—
	$	(22,358	)	$	(37,630	)

Schedule of Effective Income Tax Rate Reconciliation

A reconciliation of the statutory U.S. federal rate to the Company’s effective tax rate is as follows (in thousands):



	Year Ended December 31,
	2017			2018
Tax at federal statutory rate	$	(7,602	)	$	(7,902	)
State taxes, net of federal benefit	(1,155		)	(1,582		)
Permanent differences	535			289
Loss on Series C warrant liability	—			2,500
Tax Cut and Jobs Act	12,456			—
Change in valuation allowance	(3,832		)	6,197
General business credits	(465		)	136
Other	69			376
Provision for taxes	$	6		$	14

Schedule of Deferred Tax Assets and Liabilities

Significant components of the Company’s net deferred tax assets as of December 31, 2017 and 2018 consist of the following (in thousands):



	December 31,
	2017			2018
Deferred tax assets:
Net operating loss carryforwards	$	28,056		$	33,815
Research and development credits	5,081			4,944
Capitalized start-up costs/Intangibles	19			16
Accruals and reserves	705			1,169
Property and equipment	44			82
Stock-based compensation	197			274
Total deferred tax assets	34,102			40,300
Less: Valuation allowance	(34,102		)	(40,300		)
Net deferred tax assets	$	—		$	—

Reconciliation of Unrecognized Tax Benefits

A reconciliation of the unrecognized tax benefits from January 1, 2017 to December 31, 2018 is as follows (in thousands):



	December 31,
	2017		2018
Balance at the beginning of year	$	551	$	615
Increases related to current years’ tax positions	64		118
Increases/(decreases) related to prior years’ tax positions	—		615
Balance at end of year	$	615	$	1,348

Formation and Business of the Company (Details) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 13, 2019	Apr. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
Subsequent Event [Line Items]
Accumulated deficit			$ 163,269		$ 139,111	$ 101,556	$ 101,469
Cash and cash equivalents			15,509		24,990	33,331
Reverse stock split	0.3704
Payments of deferred offering costs			$ 1,149	$ 0	$ 233	$ 0
IPO
Subsequent Event [Line Items]
Stock issued, price per share (in USD per share) \| $ / shares			$ 20.00		$ 20.00
IPO \| Subsequent Event
Subsequent Event [Line Items]
Issued and sold common stock (in shares) \| shares		6,000,000
Stock issued, price per share (in USD per share) \| $ / shares		$ 20.00
Net proceeds		$ 108,764
IPO - Underwriting Discounts And Commissions \| Subsequent Event
Subsequent Event [Line Items]
Payments of deferred offering costs		8,400
IPO - Other Expenses \| Subsequent Event
Subsequent Event [Line Items]
Payments of deferred offering costs		$ 2,836

Summary of Significant Accounting Policies - Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 15,509	$ 24,990		$ 33,331
Restricted cash	310	310		510
Total cash, cash equivalents and restricted cash	$ 15,819	25,300	$ 29,020	33,841	$ 12,380
Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash		$ 310		310
Certificates of Deposit
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash				$ 200

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	5 years

Summary of Significant Accounting Policies - Deferred Initial Public Offering Costs (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Deferred initial public offering costs	$ 2,585,000	$ 950,000	$ 0

Summary of Significant Accounting Policies - Leases (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset	$ 3,820
Operating lease liabilities	$ 4,997
Adjustments Due to ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset		$ 3,982
Operating lease liabilities		$ 5,190

Summary of Significant Accounting Policies - ASC 842 Impact on Condensed Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other non-current assets	$ 6,460	$ 3,982	$ 1,029		$ 17
Accrued liabilities	7,658	721	7,586	$ 3,113	3,109
Other liabilities	$ 4,285	4,469	1,069		$ 0
Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other non-current assets			0
Accrued liabilities			139
Other liabilities			$ 1,069
Restatement Adjustment \| Adjustments Due to ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other non-current assets		3,982
Accrued liabilities		582
Other liabilities		$ 3,400

Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Unbilled receivables	$ 131	$ 128

Summary of Significant Accounting Policies - Cumulative Effect of Adoption of Topic 606 (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018
Balance sheet:
Accounts receivable, net	$ 4,096		$ 4,520	$ 5,215		$ 5,351
Inventories	8,056		5,744	3,248		3,202
Accrued liabilities	7,658		7,586	3,109	$ 721	3,113
Accumulated deficit	(163,269)		(139,111)	(101,556)		(101,469)
Statement of operations and comprehensive loss:
Revenue	12,766	$ 5,706	34,557	14,258
Cost of goods sold	$ 3,339	$ 1,934	10,874	5,129
Difference between Revenue Guidance in Effect before and after Topic 606 \| Accounting Standards Update 2014-09
Balance sheet:
Accounts receivable, net			26			136
Inventories			(22)			(46)
Accrued liabilities			0			4
Accumulated deficit			(4)			$ 87
Statement of operations and comprehensive loss:
Revenue			(26)
Cost of goods sold			22
Calculated under Revenue Guidance in Effect before Topic 606
Balance sheet:
Accounts receivable, net			4,494	5,215
Inventories			5,766	3,248
Accrued liabilities			7,586	3,109
Accumulated deficit			(139,107)	$ (101,556)
Statement of operations and comprehensive loss:
Revenue			34,583
Cost of goods sold			$ 10,852

Summary of Significant Accounting Policies - Advertising Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Advertising expense	$ 186	$ 218

Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - Accounting Standards Update 2016-09 $ in Thousands	Jan. 01, 2018 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting treatment	$ 0
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting treatment	$ (13)

Summary of Significant Accounting Policies - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Nett loss attributable to Silk Road Medical, Inc. common stockholders	$ (24,158)	$ (5,408)	$ (37,629)	$ (19,356)
Weighted average common stock outstanding used to compute net loss per share, basic and diluted (in shares)	1,200,719	739,308	960,882	434,158
Net loss per share, basic and diluted (in USD per share)	$ (20.12)	$ (7.31)	$ (39.16)	$ (44.58)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in Calculation of Earnings per Share (Details) - shares	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	28,051,854	28,337,489	28,277,596	28,214,582
Redeemable convertible preferred stock outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	21,238,105	21,233,190	21,233,190	21,233,190
Redeemable convertible preferred stock warrants outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	2,667,587	2,672,502	2,672,502	2,672,502
Common stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	4,138,635	4,431,797	4,364,377	4,308,890
Common stock warrants outstanding
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)	7,527	0	7,527	0

Summary of Significant Accounting Policies - Computation of Unaudited Pro Forma Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Nett loss attributable to Silk Road Medical, Inc. common stockholders	$ (24,158)	$ (5,408)	$ (37,629)	$ (19,356)
Comprehensive loss attributable to Silk Road Medical, Inc. common stockholders	(24,158)	$ (5,408)	(37,629)	$ (19,356)
Adjust: Change in fair value of redeemable convertible preferred stock warrants	$ 15,712		$ 11,906
Denominator:
Weighted average common shares used to compute net loss per share attributable to Silk Road Medical, Inc. common stockholders, basic and diluted (in shares)	1,200,719	739,308	960,882	434,158
Adjust: Conversion of redeemable convertible preferred stock (in shares)	21,233,190		21,233,190
Adjust: Cash and net exercise of redeemable convertible preferred stock warrants into common stock warrants (in shares)	1,941,105		1,940,450
Adjust: Cash and net exercise of common stock warrants into common stock (in shares)	5,970		246
Weighted average common shares used to compute pro forma net loss per share, basic and diluted (in shares)	24,380,984		24,134,768
Weighted average common shares used to compute pro forma net loss per share, basic and diluted (in shares)	24,380,984		24,134,768
Pro forma net loss per share, basic and diluted (in USD per share)	$ (0.35)		$ (1.07)
Pro forma net loss per share, basic and diluted (in USD per share)	$ (0.35)		$ (1.07)
Pro forma
Numerator:
Nett loss attributable to Silk Road Medical, Inc. common stockholders	$ (8,446)		$ (25,723)
IPO
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Stock issued, price per share (in USD per share)	$ 20.00		$ 20.00

Summary of Significant Accounting Policies - Segment and Geographical Information (Details) - segment	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Number of reportable segments	1	1
Number of operating segments	1	1

Recent Accounting Pronouncements - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash used in investing activities	$ 117	$ 455	$ 2,270	$ 443
Net change in cash, cash equivalents and restricted cash	$ (9,481)	$ (4,821)	(8,541)	21,461
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash used in investing activities			200	310
Net change in cash, cash equivalents and restricted cash			$ 200,000	$ 510

Fair Value Measurements - Narrative (Details) $ / shares in Units, $ in Thousands	Mar. 31, 2019 $ / shares	Dec. 31, 2018 $ / shares	Dec. 31, 2017 $ / shares	Apr. 30, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Oct. 31, 2015 $ / shares shares
Common Stock
Class of Stock [Line Items]
Fair value of stock (in USD per share) \| $ / shares		$ 11.29	$ 4.78
Preferred Stock Warrants
Class of Stock [Line Items]
Temporary equity, shares issued (in shares)				2,672,502
Warrant exercise price (in USD per share) \| $ / shares	$ 6.11	$ 6.11		$ 6.11	$ 6.11	$ 6.11
Warrant liability \| $				$ 144	$ 4,879
Warrant term	8 years	8 years		7 years 6 months	8 years	8 years
Preferred Stock Warrants \| Measurement Input, Price Volatility
Class of Stock [Line Items]
Risk-free interest rate				0.50	0.52	0.52
Preferred Stock Warrants \| Risk-free interest rate
Class of Stock [Line Items]
Risk-free interest rate				0.0164	0.0224	0.0194
October 2015 Preferred Stock Warrants
Class of Stock [Line Items]
Temporary equity, shares issued (in shares)						1,395,468
December 2015 Preferred Stock Warrants
Class of Stock [Line Items]
Temporary equity, shares issued (in shares)					5,324
April 2016 Preferred Stock Warrants
Class of Stock [Line Items]
Temporary equity, shares issued (in shares)				42,608
Warrant exercise price (in USD per share) \| $ / shares				$ 6.11

Fair Value Measurements - Fair Value Assumptions (Details) - Recurring - Level 3	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Time to liquidity (years)	17 days	1 year 9 months
Expected volatility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input	0.625	0.500
Discounted cash flow rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input	0.120	0.180
Risk-free interest rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input	0.026	0.019
Marketability discount rate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Measurement input	0.14	0.27

Fair Value Measurements - Measurement of Financial Liabilities (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liabilities
Redeemable convertible warrant liability	$ 31,803	$ 16,091	$ 4,185
Level 1
Liabilities
Redeemable convertible warrant liability	0	0	0
Level 2
Liabilities
Redeemable convertible warrant liability	0	0	0
Level 3
Liabilities
Redeemable convertible warrant liability	$ 31,803	$ 16,091	$ 4,185

Fair Value Measurements - Changes in Redeemable Convertible Warrant Liability (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 16,091	$ 4,185
Change in fair value recorded in other income (expense), net	15,712	11,906	$ (2,958)
Ending balance	$ 31,803	$ 16,091	$ 7,143

Balance Sheet Components - Schedule of Inventory (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Raw materials	$ 2,147	$ 1,054		$ 506
Finished products	5,909	4,690		2,742
Total inventory	$ 8,056	$ 5,744	$ 3,202	$ 3,248

Balance Sheet Components - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Work-in-process inventories	$ 0	$ 0	$ 0
Depreciation and amortization	$ 517,000	$ 129,000

Balance Sheet Components - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 3,788	$ 1,729
Less: Accumulated depreciation and amortization		(946)	(1,303)
Property and equipment, net	$ 2,846	2,880	486
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment, gross		517	76
Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross		1,217	1,059
Software
Property, Plant and Equipment [Line Items]
Property and equipment, gross		76	405
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross		1,978	189
Construction-in-progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 38	$ 60

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll and related expenses			$ 5,157		$ 2,718
Accrued professional services			1,014		64
Accrued royalty expense			313		0
Accrued travel expenses			270		68
Accrued clinical expenses			244		183
Accrued other expenses			588		76
Accrued liabilities	$ 7,658	$ 721	$ 7,586	$ 3,113	$ 3,109

Long-term Debt - Term Loan Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2017	Oct. 31, 2015
Debt Instrument [Line Items]
Term loan agreement amount	$ 15,000,000
Debt drawn down		$ 63,612,000			$ 64,396,000
Amount drawn down under the term loan agreement	15,000,000
Minimum liquidity amount					3,000,000		$ 3,000,000
Minimum net revenue target					15,000,000	$ 5,000,000	$ 1,000,000
Equity cure period		90 days
Long-term debt		$ 44,597,000			$ 44,201,000
Scenario, Forecast
Debt Instrument [Line Items]
Minimum net revenue target			$ 40,000,000	$ 30,000,000
Period One
Debt Instrument [Line Items]
Debt prepayment premium percent		8.00%			8.00%
Period Two
Debt Instrument [Line Items]
Debt prepayment premium percent		4.00%			4.00%
Period Three
Debt Instrument [Line Items]
Debt prepayment premium percent		2.00%			2.00%
Term Loan
Debt Instrument [Line Items]
Term loan agreement amount	$ 25,000,000								$ 30,000,000
Stated interest rate	10.75%
Interest rate paid-in-kind	2.75%
Interest rate paid in cash	8.00%
Percentage of outstanding loans	25.00%
Market capitalization (greater than)	$ 250,000,000
Term Loan, Tranche A
Debt Instrument [Line Items]
Term loan agreement amount									$ 20,000,000
Financing fee									1.75%
Facility fee									5.00%
Term Loan, Tranche B
Debt Instrument [Line Items]
Term loan agreement amount									$ 10,000,000
Debt drawn down								$ 5,000,000

Long-term Debt - Future Maturities Under the Term Loan (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Disclosure [Abstract]
2019	$ 2,696	$ 3,566
2020	3,677	3,677
2021	3,771	3,771
2022	52,510	52,510
Long-term debt, gross before accretion of closing fees	62,654	63,524
Add: Accretion of closing fees	958	872
Long-term debt, gross	63,612	64,396
Less: Amount representing interest	(18,841)	(20,010)
Less: Amount representing debt discount and debt issuance costs	(174)	(185)
Present value of minimum payments	$ 44,597	$ 44,201

Long-term Debt - Series C Narrative (Details) - $ / shares	1 Months Ended
	Jul. 31, 2017	Oct. 31, 2015	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Apr. 30, 2016	Dec. 31, 2015
Preferred Stock Warrants
Class of Stock [Line Items]
Warrants outstanding (in shares)		163,877	2,667,587	2,672,502	2,672,502
Warrant exercise price (in USD per share)		$ 6.11	$ 6.11	$ 6.11		$ 6.11	$ 6.11
Series C
Class of Stock [Line Items]
Temporary equity, shares issued (in shares)	163,877	327,759
Stock issued, price per share (in USD per share)	$ 6.11	$ 6.11

Commitments and Contingencies - Lease Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Nov. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Leasehold improvements incentive		$ 794
Term of contract		6 years
Renewal term		5 years
Leasehold improvements financing		$ 316
Leasehold improvements financing interest rate		7.00%
Operating lease costs	$ 217
Operating lease liabilities	$ 193
Weighted average discount rate	6.50%
Weighted average remaining lease term	5 years 7 months 6 days

Commitments and Contingencies - Balance Sheet Information (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Operating lease right-of-use asset in other non-current assets	$ 3,820
Operating lease liability in accrued liabilities	712
Operating lease liability in other liabilities	4,285
Total operating lease liabilities	$ 4,997

Commitments and Contingencies - Operating Lease Maturities (Details) $ in Thousands	Mar. 31, 2019 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 760
2020	1,037
2021	1,066
2022	1,096
2023	1,127
2024	904
Total lease payments	5,990
Less: imputed interest	(993)
Present value of lease liabilities	$ 4,997

Commitments and Contingencies - Topic 840 Aggregate Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 1,002
2020	1,002
2021	1,031
2022	1,044
2023 and thereafter	1,920
Total Minimum Lease Payments	$ 5,999

Commitments and Contingencies - Purchase Obligation Narrative (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Purchase obligation	$ 2,855,100	$ 4,648,000

Redeemable Convertible Preferred Stock - Schedule of Redeemable Convertible Preferred Stock (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity [Line Items]
Shares Authorized (in shares)	24,069,615	24,069,615		24,069,615
Shares Issued (in shares)	21,238,105	21,233,190		21,233,190
Shares Outstanding (in shares)	21,238,105	21,233,190	21,233,190	21,233,190	14,350,216
Preferential Liquidation Value (in thousands)	$ 121,174,000	$ 121,144,000		$ 121,144,000
Carrying Value (in thousands)	$ 105,265,000	$ 105,235,000	$ 105,235,000	$ 105,235,000	$ 63,417,000
Series A
Temporary Equity [Line Items]
Shares Authorized (in shares)	1,629,629	1,629,629		1,629,629
Shares Issued (in shares)	1,629,626	1,629,626		1,629,626
Shares Outstanding (in shares)	1,629,626	1,629,626		1,629,626
Per share Preference (in USD per share)	$ 2.70	$ 2.70		$ 2.70
Preferential Liquidation Value (in thousands)	$ 4,400,000	$ 4,400,000		$ 4,400,000
Carrying Value (in thousands)	$ 4,369,000	$ 4,369,000		$ 4,369,000
Series A-1
Temporary Equity [Line Items]
Shares Authorized (in shares)	1,111,111	1,111,111		1,111,111
Shares Issued (in shares)	1,111,109	1,111,109		1,111,109
Shares Outstanding (in shares)	1,111,109	1,111,109		1,111,109
Per share Preference (in USD per share)	$ 3.38	$ 3.38		$ 3.38
Preferential Liquidation Value (in thousands)	$ 3,755,000	$ 3,755,000		$ 3,755,000
Carrying Value (in thousands)	$ 3,723,000	$ 3,723,000		$ 3,723,000
Series B
Temporary Equity [Line Items]
Shares Authorized (in shares)	6,264,470	6,264,470		6,264,470
Shares Issued (in shares)	6,264,463	6,264,463		6,264,463
Shares Outstanding (in shares)	6,264,463	6,264,463		6,264,463
Per share Preference (in USD per share)	$ 6.11	$ 6.11		$ 6.11
Preferential Liquidation Value (in thousands)	$ 38,276,000	$ 38,276,000		$ 38,276,000
Carrying Value (in thousands)	$ 38,014,000	$ 38,014,000		$ 38,014,000
Series C
Temporary Equity [Line Items]
Shares Authorized (in shares)	15,064,405	15,064,405		15,064,405
Shares Issued (in shares)	12,232,907	12,227,992		12,227,992
Shares Outstanding (in shares)	12,232,907	12,227,992		12,227,992
Per share Preference (in USD per share)	$ 6.11	$ 6.11		$ 6.11
Preferential Liquidation Value (in thousands)	$ 74,743,000	$ 74,713,000		$ 74,713,000
Carrying Value (in thousands)	$ 59,159,000	$ 59,129,000		$ 59,129,000

Redeemable Convertible Preferred Stock - Narrative (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($) director $ / shares shares	Dec. 31, 2018 USD ($) director $ / shares shares	Dec. 31, 2017 $ / shares shares	Apr. 30, 2016 $ / shares	Dec. 31, 2015 $ / shares	Oct. 31, 2015 $ / shares shares
Temporary Equity [Line Items]
Dividends declared \| $	$ 0	$ 0
Conversion, minimum offering price (in USD per share)	$ 18.31	$ 18.31
Conversion, minimum offering price \| $	$ 50,000,000	$ 50,000,000
Conversion ratio (in shares) \| shares	1	1
Preferred Stock Warrants
Temporary Equity [Line Items]
Percentage of shares purchased available for warrant holders to purchase	50.00%
Warrant exercise price (in USD per share)	$ 6.11	$ 6.11		$ 6.11	$ 6.11	$ 6.11
Time to liquidity (years)	8 years	8 years		7 years 6 months	8 years	8 years
Warrants outstanding (in shares) \| shares	2,667,587	2,672,502	2,672,502			163,877
Series A
Temporary Equity [Line Items]
Number of directors each holders is entitled to elect \| director	2	2
Dividend rate (in USD per share)	$ 0.22	$ 0.22
Per share Preference (in USD per share)	2.70	2.70	$ 2.70
Conversion price (in USD per share)	2.70	2.70
Series A-1
Temporary Equity [Line Items]
Dividend rate (in USD per share)	0.27	0.27
Per share Preference (in USD per share)	3.38	3.38	3.38
Conversion price (in USD per share)	$ 3.38	$ 3.38
Series B
Temporary Equity [Line Items]
Number of directors each holders is entitled to elect \| director	3	3
Dividend rate (in USD per share)	$ 0.49	$ 0.49
Per share Preference (in USD per share)	6.11	6.11	6.11
Conversion price (in USD per share)	$ 6.11	$ 6.11
Series C
Temporary Equity [Line Items]
Number of directors each holders is entitled to elect \| director	2	2
Dividend rate (in USD per share)	$ 0.49	$ 0.49
Per share Preference (in USD per share)	6.11	6.11	$ 6.11
Conversion price (in USD per share)	$ 6.11	$ 6.11

Common Stock - Narrative (Details)	3 Months Ended	12 Months Ended
Common Stock - Narrative (Details)	Mar. 31, 2019 USD ($) vote $ / shares shares	Dec. 31, 2018 USD ($) vote $ / shares shares	Dec. 17, 2018 $ / shares shares	Dec. 31, 2017 $ / shares shares
Class of Warrant or Right [Line Items]
Shares authorized (in shares)	29,879,220	29,879,220		29,879,220
Par value (in USD per share) \| $ / shares	$ 0.001	$ 0.001		$ 0.001
Shares issued (in shares)	1,386,615	1,135,310		663,270
Shares outstanding (in shares)	1,386,615	1,135,310		663,270
Dividends declared \| $	$ 0	$ 0
Number of votes entitled per share of common stock \| vote	1	1
Warrants to Purchase Common Stock
Class of Warrant or Right [Line Items]
Number of shares which may be purchased with warrants (in shares)		7,527	7,527
Warrant exercise price (in USD per share) \| $ / shares			$ 8.27
Warrants outstanding (in shares)	7,527	7,527

Common Stock - Shares Reserved for Future Issuance (Details) - shares	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	28,248,103	28,499,408	28,706,795
Warrants to purchase common stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	7,527	7,527	0
Exercise of options under stock plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	4,138,635	4,364,377	4,308,890
Issuance of options under stock plan
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	32,326	57,889	328,290
Conversion of Series A convertible preferred stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	1,629,629	1,629,629	1,629,629
Conversion of Series A-1 convertible preferred stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	1,111,111	1,111,111	1,111,111
Conversion of Series B convertible preferred stock
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	6,264,470	6,264,470	6,264,470
Conversion of Series C convertible preferred stock and warrants
Class of Stock [Line Items]
Common stock reserved for future issuance (in shares)	15,064,405	15,064,405	15,064,405

Stock Option Plan - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)			32,950			629,716	1,375,329
Aggregate intrinsic value of options exercised			$ 2,461
Compensation expense for share-based awards			$ 262	$ 162	$ 911	$ 535
Common stock reserved for future issuance (in shares)	28,248,103	28,499,408	28,248,103		28,499,408	28,706,795
Compensation expensed not yet recognized	$ 2,433	$ 2,524	$ 2,433		$ 2,524
Compensation expensed not yet recognized, period for recognition			3 years 14 days		3 years 5 months 19 days
ISO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise threshold as a percentage of fair value of shares	100.00%	100.00%	100.00%		100.00%
NSO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise threshold as a percentage of fair value of shares	85.00%	85.00%	85.00%		85.00%
ISO and NSO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option term			10 years		10 years
Option vesting term			4 years		4 years
Monthly vesting period			3 years		3 years
ISO and NSO \| Vesting Term One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage			25.00%		25.00%
ISO and NSO \| 10% Stockholders
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise threshold as a percentage of fair value of shares	110.00%	110.00%	110.00%		110.00%
Common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)					575,314	1,371,626
Weighted average grant date fair value (in USD per share)					$ 2.81	$ 0.83
Fair value of options vested (in USD per share)					$ 569,000	$ 423,000
Aggregate intrinsic value of options exercised					$ 787	$ 143
Compensation expense for share-based awards					$ 728	$ 442
Percent of outstanding shares of common stock	1.00%
Percent of purchase of price of common stock	85.00%
Common stock reserved for future issuance (in shares)	434,000		434,000
Number of additional shares allowable under the plan (in shares)	1,200,000		1,200,000
Nonemployee options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)					52,960	3,703
Nonemployee stock-based compensation expense					$ 183	$ 93
2007 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance (in shares)	5,488,229	5,488,229	5,488,229		5,488,229
NeuroCo Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance (in shares)	0	0	0		0
Options granted (in shares)		1,442			1,442
2019 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock reserved for issuance (in shares)	2,317,000		2,317,000
Percent of outstanding shares of common stock			4.00%
2019 Stock Option Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of additional shares allowable under the plan (in shares)	3,000,000		3,000,000

Stock Option Plan - Activity Under Compensation Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares Available for Grant
Beginning balance (in shares)	57,889	328,290	27,605
Authorized (in shares)	0		223,664	1,421,867
Options granted (in shares)	(32,950)		(629,716)	(1,375,329)
Options cancelled (in shares)	(7,387)		(135,651)	(254,147)
Ending balance (in shares)	32,326	57,889	328,290	27,605
Number of Shares
Beginning balance (in shares)	4,364,377	4,308,890	3,478,346
Options granted (in shares)	(32,950)		(629,716)	(1,375,329)
Options exercised (in shares)	(251,305)		(438,578)	(290,638)
Options cancelled (in shares)	(7,387)		(135,651)	(254,147)
Ending balance (in shares)	4,138,635	4,364,377	4,308,890	3,478,346
Weighted Average Exercise Price
Beginning balance (in USD per share)	$ 3.79	$ 3.09	$ 1.46
Options granted (in USD per share)	11.29		6.51	$ 6.54
Options exercised (in USD per share)	1.50		1.50	1.17
Options cancelled (in USD per share)	1.56		1.56	1.55
Ending balance (in USD per share)	$ 4.00	$ 3.79	$ 3.09	$ 1.46
Weighted Average Remaining Contractual Term (in Years), awards outstanding	7 years 3 months 3 days	7 years 4 months 9 days	7 years 9 months 21 days	7 years 8 months 15 days
Aggregate Intrinsic Value, awards outstanding	$ 30,602	$ 33,132	$ 5,073	$ 444
Number of Shares, vested and exercisable (in shares)	2,451,044	2,459,116
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 2.70	$ 2.35
Weighted Average Remaining Contractual Term (in Years), vested and exercisable	6 years 4 months 9 days	6 years 3 months 10 days
Aggregate Intrinsic Value, vested and exercisable	$ 21,195	$ 22,109
Number of Shares, vested and expect to vest (in shares)	4,138,635	4,464,377
Weighted Average Exercise Price, vested and expected to vest (in dollars per share)	$ 4.00	$ 3.79
Weighted Average Remaining Contractual Term (in Years), vested and expected to vest	7 years 3 months 3 days	7 years 4 months 9 days
Aggregate Intrinsic Value, vested and expected to vest	$ 30,602	$ 33,132

Stock Option Plan - Information by Exercise Price (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	4,138,635	4,364,377	4,308,890	3,478,346
Weighted Average Remaining Contractual Term (in Years)	7 years 3 months 3 days	7 years 4 months 9 days	7 years 9 months 21 days	7 years 8 months 15 days
Weighted Average Exercise Price (in USD per share)	$ 4.00	$ 3.79	$ 3.09	$ 1.46
Number Exercisable (in shares)	2,451,044	2,459,116
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 2.70	$ 2.35
$0.68
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		101,848
Weighted Average Remaining Contractual Term (in Years)		1 year 7 months 2 days
Weighted Average Exercise Price (in USD per share)		$ 0.68
Number Exercisable (in shares)		101,848
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 0.68
$1.35
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		107,938
Weighted Average Remaining Contractual Term (in Years)		2 years 9 months
Weighted Average Exercise Price (in USD per share)		$ 1.35
Number Exercisable (in shares)		107,938
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.35
$1.38
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		304,396
Weighted Average Remaining Contractual Term (in Years)		3 years 11 months 15 days
Weighted Average Exercise Price (in USD per share)		$ 1.38
Number Exercisable (in shares)		304,396
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.38
$1.46
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		385,070
Weighted Average Remaining Contractual Term (in Years)		6 years 2 months 15 days
Weighted Average Exercise Price (in USD per share)		$ 1.46
Number Exercisable (in shares)		360,470
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.46
$1.57
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		97,311
Weighted Average Remaining Contractual Term (in Years)		8 years 25 days
Weighted Average Exercise Price (in USD per share)		$ 1.57
Number Exercisable (in shares)		42,013
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.57
$1.60
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		1,463,632
Weighted Average Remaining Contractual Term (in Years)		6 years 9 months 25 days
Weighted Average Exercise Price (in USD per share)		$ 1.60
Number Exercisable (in shares)		1,128,569
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.60
$3.16
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		462,764
Weighted Average Remaining Contractual Term (in Years)		8 years 11 months 12 days
Weighted Average Exercise Price (in USD per share)		$ 3.16
Number Exercisable (in shares)		162,549
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 3.16
$4.73
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		332,385
Weighted Average Remaining Contractual Term (in Years)		8 years 11 months 1 day
Weighted Average Exercise Price (in USD per share)		$ 4.73
Number Exercisable (in shares)		86,506
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 4.73
$6.11
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		552,462
Weighted Average Remaining Contractual Term (in Years)		9 years 3 months 25 days
Weighted Average Exercise Price (in USD per share)		$ 6.11
Number Exercisable (in shares)		29,848
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 6.11
$8.27
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		78,941
Weighted Average Remaining Contractual Term (in Years)		9 years 11 months 4 days
Weighted Average Exercise Price (in USD per share)		$ 8.27
Number Exercisable (in shares)		1,527
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 8.27
$12.15
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		477,630
Weighted Average Remaining Contractual Term (in Years)		8 years 11 months 1 day
Weighted Average Exercise Price (in USD per share)		$ 12.15
Number Exercisable (in shares)		133,452
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 12.15

Stock Option Plan - Stock Option Assumptions (Details)	3 Months Ended		12 Months Ended
Stock Option Plan - Stock Option Assumptions (Details)	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, maximum			38.80%	41.00%
Common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 3 months	6 years 3 months	6 years 3 months
Expected volatility	42.70%	38.10%
Expected volatility, minimum			38.10%	39.00%
Risk-free interest rate	2.54%	2.68%
Risk-free interest rate, minimum			2.68%	1.03%
Risk-free interest rate, maximum			2.98%	2.25%
Dividend yield	0.00%	0.00%	0.00%	0.00%
Nonemployee options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonemployee dividend yield			0.00%	0.00%
Minimum \| Common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)				1 year
Minimum \| Nonemployee options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonemployee expected term (in years)			5 years	3 years 9 months
Nonemployee expected volatility			38.00%	39.00%
Nonemployee risk free interest rate			2.71%	2.06%
Maximum \| Common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)				6 years 3 months
Maximum \| Nonemployee options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonemployee expected term (in years)			6 years 3 months	9 years 9 months
Nonemployee expected volatility			38.80%	56.00%
Nonemployee risk free interest rate			2.90%	2.39%

Stock Option Plan - Stock-based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 262	$ 162	$ 911	$ 535
Cost of goods sold
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	15	13	51	49
Research and development expenses
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	27	29	256	98
Selling, general and administrative expenses
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 220	$ 120	$ 604	$ 388

Income Taxes - Components of Income Before Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
United States	$ (37,630)	$ (22,358)
International	0	0
Total	$ (37,630)	$ (22,358)

Income Taxes - Reconciliation of U.S Statutory Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Tax at federal statutory rate	$ (7,902)	$ (7,602)
State taxes, net of federal benefit	(1,582)	(1,155)
Permanent differences	289	535
Loss on Series C warrant liability	2,500	0
Tax Cut and Jobs Act		12,456
Change in valuation allowance	6,197	(3,832)
General business credits	136	(465)
Other	376	69
Provision for taxes	$ 14	$ 6

Income Taxes - Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 33,815	$ 28,056
Research and development credits	4,944	5,081
Capitalized start-up costs/Intangibles	16	19
Accruals and reserves	1,169	705
Property and equipment	82	44
Stock-based compensation	274	197
Total deferred tax assets	40,300	34,102
Less: Valuation allowance	(40,300)	(34,102)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
Income Taxes - Narrative (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Increase (decrease) in deferred tax asset valuation allowance	$ 6,197,000	$ (3,832,000)
Unrecognized tax benefits	1,348,000	615,000	$ 551,000
Unrecognized tax benefits that would impact effective tax rate	0
Unrecognized tax benefits, income tax penalties and interest expense	0	$ 0
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	125,187,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	115,827,000
Research Tax Credit Carryforward \| Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Research and development tax credits	4,083,000
Research Tax Credit Carryforward \| State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Research and development tax credits	$ 2,655,000

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at the beginning of year	$ 615	$ 551
Increases related to current years’ tax positions	118	64
Increases/(decreases) related to prior years’ tax positions	615	0
Balance at end of year	$ 1,348	$ 615

Acquisition of Variable Interest Entity - NeuroCo (Details) - USD ($)		1 Months Ended	12 Months Ended
	Dec. 17, 2018	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2017
Business Acquisition [Line Items]
Carrying amount of the non-controlling interest	$ 0
NeuroCo, Inc
Business Acquisition [Line Items]
Equity interest acquired	100.00%
Shares issued as consideration (in shares)	33,462
Consideration given, amount of notes payable forgiven	$ 1,600,000
NeuroCo, Inc
Business Acquisition [Line Items]
Promissory note amount		$ 498,000		$ 1,544,000
Notes receivable, interest rate		2.74%
Notes receivable, term		10 years
NeuroCo common stock issuance			$ 154,000

401(k) Plan (Details)	12 Months Ended
401(k) Plan (Details)	Dec. 31, 2018 USD ($)
Retirement Benefits [Abstract]
Percentage employee contribution	90.00%
Employer discretionary contribution	$ 0

Subsequent Events - Narrative (Details) - $ / shares	1 Months Ended	3 Months Ended	12 Months Ended
Subsequent Events - Narrative (Details) - $ / shares	Feb. 28, 2019	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2019	May 01, 2019	Apr. 30, 2019	Apr. 03, 2019	Sep. 30, 2018
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)		32,950		629,716	1,375,329
Exercise price (in USD per share)		$ 11.29		$ 6.51	$ 6.54
Shares authorized (in shares)		29,879,220	29,879,220	29,879,220
Common stock, par value (in USD per share)		$ 0.001	$ 0.001	$ 0.001
IPO
Subsequent Event [Line Items]
Stock issued, price per share (in USD per share)		$ 20.00	$ 20.00
Term Loan
Subsequent Event [Line Items]
Stated interest rate										10.75%
Interest rate paid in cash										8.00%
Interest rate paid-in-kind										2.75%
Employee Stock Option
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)			575,314	1,371,626
2007 Stock Option Plan \| Employee Stock Option
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)	32,950
Exercise price (in USD per share)	$ 11.29
Grant date fair value (in USD per share)	$ 160,000
Requisite service period	4 years
2019 Stock Option Plan
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)		2,317,000
Subsequent Event
Subsequent Event [Line Items]
Shares authorized (in shares)								100,000,000
Common stock, par value (in USD per share)								$ 0.001
Preferred shares authorized (in shares)								5,000,000
Preferred shares (in USD per share)								$ 0.001
Subsequent Event \| IPO
Subsequent Event [Line Items]
Stock issued, price per share (in USD per share)								$ 20.00
Subsequent Event \| Term Loan
Subsequent Event [Line Items]
Stated interest rate							10.00%
Interest rate paid in cash							8.00%
Interest rate paid-in-kind							2.00%
Subsequent Event \| 2019 Stock Option Plan
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)						36,982			687,176
Weighted average exercise price (in dollars per share)						$ 47.54

Events Subsequent to Original Issuance of Condensed Consolidated Financial Statements (unaudited) (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2019	Apr. 03, 2019
Subsequent Event [Line Items]
Exercise price (in USD per share)	$ 11.29	$ 6.51	$ 6.54
2019 Stock Option Plan
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)	2,317,000
2019 Stock Option Plan \| Subsequent Event
Subsequent Event [Line Items]
Shares of common stock reserved for issuance (in shares)				36,982	687,176

Label	Element	Value
Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 87,000
Accounting Standards Update 2014-09 [Member] \| Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	87,000
Accounting Standards Update 2016-09 [Member] \| Additional Paid-in Capital [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 13,000