Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Schedule of Accounts, Notes, Loans and Financing Receivable

The allowance for doubtful accounts as of December 31, 2018 and 2017, comprised the following (in thousands):

	Allowance for doubtful accounts
Balance at December 31, 2016	$	2,740
Provision for doubtful accounts	14
Balance at December 31, 2017	$	2,754
Provision for doubtful accounts	23
Write off of uncollectible accounts	(2,702		)
Balance at December 31, 2018	$	75

Estimated Useful Lives of Property and Equipment

Property, equipment and leasehold improvements are recorded at cost and depreciated using the straight-line method over the estimated useful lives of the assets, which are:

Plant and Machinery	3 – 5 years
Instruments	4 – 5 years
Office equipment	3 – 7 years
Leasehold improvements	over the shorter of the remaining life of the lease or the useful economic life of the asset

Schedule of Product Warranty Liability

Product warranty reserve activity for the years ended December 31, 2018, 2017 and 2016 is as follows (in thousands):

	2018			2017			2016
Beginning balance	$	470		$	219		$	118
Warranty expenses incurred	(1,495		)	(1,160		)	(421		)
Provisions	1,355			1,411			522
Ending balance	$	330		$	470		$	219

Schedule of Weighted Average Number of Shares

The calculations of diluted net loss per share for the years ended December 31, 2018, 2017 and 2016 did not include the effects of the following stock options or other unvested equity awards which were outstanding as of the end of each year because the inclusion of these securities would have been anti-dilutive (in thousands).

	Year Ended December 31,
	2018	2017	2016
Options outstanding to purchase common stock	2,440	2,490	2,570
Other unvested equity awards	2,994	2,307	2,000
Total	5,434	4,797	4,570

Schedules of Concentration of Risk, by Risk Factor

The following table summarizes customers who accounted for 10% or more of net accounts receivable:

	December 31,
	2018		2017
Laboratory Corporation of America, Inc.	24	%	23	%