Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (12,799)	$ (9,594)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	255	129
Stock-based compensation	412	273
Amortization of right-of-use assets	249
Loss on write down of fixed assets	19
Change in fair value of warrant liability	609	(41)
Amortization of debt issuance costs	104
Changes in operating assets and liabilities:
Accounts receivable	(1,089)	(503)
Inventory	(1,883)	(1,168)
Prepaid expenses and other current assets	(1,396)	11
Other assets	(19)	(143)
Accounts payable	369	346
Accrued and other current liabilities	991	(678)
Lease liabilities	(237)
Other liabilities		(9)
Net cash used in operating activities	(14,415)	(11,377)
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from maturities of available-for-sale securities		1,807
Purchase of property and equipment	(420)	(315)
Net cash (used in) provided by investing activities	(420)	1,492
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock upon initial public offering, net of issuance costs paid	102,977
Proceeds from issuance of common stock in private placement	10,000
Proceeds from stock option exercises	169	315
Proceeds from warrant exercises	110	101
Net cash provided by financing activities	113,256	416
Net increase (decrease) in cash, cash equivalents and restricted cash	98,421	(9,469)
Cash, cash equivalents and restricted cash at beginning of period	40,093	51,923
Cash, cash equivalents and restricted cash equivalents at end of period	138,514	42,454
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION:
Interest paid	132	18
Income tax paid	4
NON-CASH INVESTING AND FINANCING ACTIVITIES:
Common stock issued on conversion of convertible preferred stock	152,806
Issuance of Series A-1 convertible preferred stock on net exercise of warrants		211
Common stock issued upon net exercise of warrants	133
Common stock warrants issued on conversion of preferred stock warrants and the reclassification of the warrant liability	789
Deferred offering costs included in accounts payable and accrued liabilities	2,215
Right-of-use asset obtained in exchange for lease liability	73
Property and equipment purchases included in accounts payable	$ 93	74
Issuance of common stock warrants in connection with debt financing		$ 104