The fair value of the Company’s cash equivalents and short-term investments approximates their respective carrying amounts due to their short-term maturity. The following is a summary of the gross unrealized gains and unrealized losses on the Company’s investment securities (in thousands):

	December 31, 2018
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Investment Securities
Commercial paper	$	119,692	$	—	$	(90	)	$	119,602
Corporate notes		111,652		—		(120	)		111,532
Total securities	$	231,344	$	—	$	(210	)	$	231,134

	December 31, 2017
	Amortized Cost		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Aggregate Fair Value
Investment Securities
Commercial paper	$	61,167	$	—	$	(81	)	$	61,086
Corporate notes		165,712		1		(332	)		165,381
Total securities	$	226,879	$	1	$	(413	)	$	226,467

Realized gains or losses from the sale of investments and other-than-temporary impairments, if any, on available-for-sale securities are reported in other income (expense), net as incurred. The cost of securities sold was determined based on the specific identification method. The amount of realized gains and realized losses on investments for the periods presented have not been material.

The amortized costs and estimated fair values of the Company’s available-for-sale securities by contractual maturities as of December 31, 2018 were as follows (in thousands):

	Amortized Cost		Fair Value
Amounts maturing within one year	$	231,344	$	231,134
Amounts after one year through five years		—		—
Total investment securities	$	231,344	$	231,134

	December 31,
	2018		2017
Raw materials	$	37,453	$	51,602
Finished goods		54,582		46,517
Total inventories	$	92,035	$	98,119

	December 31,
	2018			2017
Laboratory equipment	$	2,874		$	2,416
Computer equipment and software		8,751			5,076
Furniture and fixtures		3,903			2,241
Leasehold improvements		4,218			1,221
Construction in process		1,811			2,734
Total		21,557			13,688
Less: Accumulated depreciation and amortization		(8,756	)		(4,869	)
Property and equipment, net	$	12,801		$	8,819

Depreciation and amortization expense for the years ended December 31, 2018, 2017 and 2016 was $3.9 million, $2.5 million and $1.7 million, respectively.

	December 31,
	2018		2017
Accrued payroll and related expenses	$	26,792	$	26,108
Accrued professional fees		1,595		4,734
Accrued taxes		2,506		2,827
Accrued clinical and research expenses		1,105		1,279
Accrued interest		243		243
Accrued warranty		1,236		708
Accrued other		5,313		3,491
Total accrued liabilities	$	38,790	$	39,390