Note 3. Revenue and Accounts Receivable

Revenue by payor type for the three and six months ended June 30, 2013 and 2012 is comprised of the following:

Accounts receivable by payor type at June 30, 2013 and December 31, 2012 consists of the following:

We have historically derived a significant portion of our revenue from a limited number of test ordering sites. Our test ordering sites are largely hospitals, cancer centers, reference laboratories and physician offices, as well as biopharmaceutical companies as part of a clinical trial. Oncologists and pathologists at these sites order the tests on behalf of the needs of their oncology patients or as part of a clinical trial sponsored by a biopharmaceutical company in which the patient is being enrolled. We generally do not have formal, long-term written agreements with such test ordering sites, and, as a result, we may lose these significant test ordering sites at any time.

The top five test ordering sites during the three months ended June 30, 2013 and 2012 accounted for 74% and 62% respectively, of our clinical testing volumes, with 23% and 52% respectively, of the volume coming from community hospitals. During the three months ended June 30, 2013, there was one site which accounted for more than 10% of our revenue: a clinical trials client accounted for approximately 50% of our revenue. During the three months ended June 30, 2012, there were three sites which each accounted for 10% or more of our clinical revenue: a university teaching center accounting for approximately 13%, a community hospital accounted for approximately 11%, and a community hospital network accounted for approximately 11%.

The top five test ordering sites during the six months ended June 30, 2013 and 2012 accounted for 69% and 61%, respectively, of our clinical testing volumes, with 27% and 48%, respectively, of the volume coming from community hospitals. During the six months ended June 30, 2013, there was one site which accounted for more than 10% of our revenue: a clinical trials client accounted for approximately 38% of our revenue. During the six months ended June 30, 2012, there were four sites which each accounted for approximately 10% or more of our clinical revenue: a university teaching center accounting for approximately 17%; a community hospital accounted for approximately 12%; and a clinical trials client and a community hospital network each accounted for approximately 11%.