Note 3. Revenue and Accounts Receivable

Revenue by payor type for the three months ended March 31, 2014 and 2013 is comprised of the following:

Accounts receivable by payor type at March 31, 2014 and December 31, 2013 consists of the following:

We have historically derived a significant portion of our revenue from a limited number of test ordering sites. The test ordering sites are largely hospitals, cancer centers, reference laboratories, physician offices and clinical trial clients. 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. The top five test ordering sites during the three months ended March 31, 2014 and 2013 accounted for 63% and 58% respectively, of our clinical testing volumes, with 32 % and 30% respectively, of the volume coming from community hospitals. During the three months ended March 31, 2014, there was one site which accounted for approximately 10% or more of our revenue. A clinical trial client accounted for approximately 33% of our revenue. During the three months ended March 31, 2013, there were three sites which each accounted for approximately 10% or more of our revenue. A clinical trial client accounted for approximately 21%, a university teaching center accounted for approximately 12%, and a community oncology practice accounted for approximately 12% of our revenue. While we have agreements with our clinical trials clients, volumes from these clients is subject to the progression and continuation of the trials as determined by the client which can impact testing volume. We generally do not have formal written agreements with other testing sites and, as a result, we may lose these significant test ordering sites at any time.