CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (17,453)	$ (23,039)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	722	1,013
Change in allowance for doubtful accounts	(5)	(36)
Stock-based compensation	2,312	1,438
Change in fair value of redeemable convertible preferred stock warrants	826	(166)
Loss on write-off of property and equipment	52	214
Write-off of debt discount	0	650
Amortization of debt discount	259	285
Write-off of public offering costs	0	1,292
Forgiveness of notes receivable	0	437
Short-term investments accretion	(209)	0
Changes in operating assets and liabilities
Accounts receivable	(1,023)	(1,313)
Inventory	(759)	(980)
Prepaid expenses and other assets	(752)	72
Accounts payable	251	811
Accrued liabilities and other	1,260	1,792
Net cash used in operating activities	(14,519)	(17,530)
Cash flows from investing activities
Purchase of property and equipment	(942)	(478)
Purchase of short-term investments	(96,883)	0
Net cash used in investing activities	(97,825)	(478)
Cash flows from financing activities
Proceeds from initial public offering, net of underwriting discounts and commissions	115,506	0
Proceeds from the exercise of common stock options, net	1,614	383
Repurchase of common stock	(73)	0
Repayment of stockholders’ notes receivable	0	84
Repayment of debt financing	0	(1,119)
Extinguishment of debt financing	0	(29,081)
Proceeds from debt financing	0	40,000
Payment of debt issuance costs	0	(1,540)
Proceeds from the issuance of redeemable convertible preferred stock, net	0	5,427
Payments of public offering costs	(1,897)	(1,292)
Net cash provided by financing activities	115,150	12,862
Effect of exchange rate changes on cash and cash equivalents	(94)	(346)
Net decrease in cash and cash equivalents	2,712	(5,492)
Cash and cash equivalents at
Beginning of period	22,408	27,900
End of period	25,120	22,408
Supplemental disclosure of cash flow information
Cash paid for interest	5,500	4,514
Supplemental disclosure of non-cash information
Conversion of redeemable convertible preferred stock to common stock	118,547	0
Conversion of preferred stock warrants to common stock warrants	1,248	0
Vesting of early exercised stock options	139	122
Purchases of property and equipment included in accounts payable and accrued liabilities	82	97
Public offering costs included in accounts payable and accrued liabilities	$ 7	$ 0