Balance Sheet Components - Components of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Payables And Accruals [Abstract]
Clinical studies	$ 1,375	$ 756
Accrued interest payable on debt	862	0
Professional fees	620	880
Debt financing fees	600	0
Test sample processing fees	524	426
Accrued overpayments and refunds	281	163
Software implementation costs	176	0
Deferred rent – current portion	374	258
Capital leases – current portion	68	71
Other accrued expenses	440	338
Total accrued and other liabilities	$ 5,320	$ 2,892