Quest Diagnostics, the nation’s largest diagnostic testing company, has announced that it will begin offering an HIV tropism test, Pathway Diagnostics’ SensiTrop assay. A tropism test, used to determine which co-receptor a person’s HIV uses to infect CD4 cells, is necessary before starting treatment with the new CCR5 entry inhibitor Selzentry (maraviroc). Until now, Monogram Bioscience’s Trofile assay was the most widely available tropism assay.

Selzentry is only effective against HIV that uses the CCR5 co-receptor on the surface of a CD4 cell. People who have HIV that uses another co-receptor, CXCR4, or who have a mix of virus that can use either or both co-receptors, will not respond to Selzentry. A tropism test assesses which co-receptors a person’s virus uses (for more information on co-receptors and Selzentry see “The Trouble With Tropism”).

According to a Quest Diagnostics representative, the new SensiTrop test will have a turnaround time of approximately seven days and should be available in their labs by spring 2008. In the mean time, Quest labs will be able to process blood samples through their partnership with the test’s originator, Pathway Diagnostics. The currently available test from Monogram takes 14 to 16 days from the time they receive the blood sample.

The currently available test from Monogram may miss some CXCR4 virus if it makes up less than 10 percent of the population of virus in the sample being tested. Though Pathway has announced that their test is more sensitive, further research will likely be needed to validate this claim.