Contract assets and liabilities result due to the timing of revenue recognition, billings and cash collections.

Contract assets include our right to payment for goods and services already transferred to a customer when the right to payment is conditional on something other than the passage of time, for example contracts where we recognize revenue over time but do not have a contractual right to payment until we complete the performance obligations. Contract assets are included in accounts receivable in our Consolidated Balance Sheet.

Contract liabilities consist of advance payments received and billings in excess of revenue recognized. We generally receive up-front payments relating to our consortia studies; we recognize revenue over the life of the study as the testing and analysis results are made available to our consortia members. We record billings in excess of revenue recognized for contracts with a duration less than twelve months as unearned revenue. We classify contract liabilities for contracts with a duration greater than twelve months as current or non-current based on the timing of when we expect to recognize revenue. The current portion of contract liabilities is included in unearned revenue and the non-current portion of contract liabilities is included in other long-term liabilities in our Consolidated Balance Sheet.

The balance of contract assets and contract liabilities consisted of the following (in thousands):

	June 30, 2019		December 31, 2018
Contract assets
Current	$	892	$	1,145
Non-Current		—		188
	$	892	$	1,333
Contract Liabilities
Current	$	6,379	$	5,963
Non-current		576		1,401
	$	6,955	$	7,364

	June 30, 2019
Estimate of when contract liabilities will be recognized as revenue
within 12 months	$	6,379
within 12 to 24 months		576
greater than 24 months		—

We did not recognize any impairment losses on our receivables and contract assets for the three and six months ended June 30, 2019.