Tropism testing is necessary if Selzentry is to be included in an antiretroviral regimen and is used to determine which co-receptor on CD4 cells a person’s HIV is using: CCR5, CXCR4 or both (dual/mixed). Selzentry is only effective against CCR5-using HIV.

Monogram’s Trofile was determined to be an effective tropism assay in clinical trials of Selzentry. However, it is expensive ($1,960 for each test) and has a 14- to 16-day turnaround time. The availability of Pathway’s SensiTrop was announced after the approval of Selzentry, boasting a lower cost and a two- to four-day wait for results. 

Long-awaited results comparing the effectiveness of SensiTrop with the validated Trofile assay are now available. The Pfizer press release reported the results of an analysis involving blood specimens collected for 100 treatment-experienced patients. Of the 39 HIV samples identified as dual/mixed- or CXCR4-tropic using Trofile, 19 were identified as CCR5-tropic using SensiTrop. “The findings,” Pfizer writes, “mean that there is a possibility that some patients whose tropism was assessed with SensiTrop may have received Selzentry based on an inaccurate R5-tropism reading.”