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Education > Clinical Updates

Clinical Update #12

News from the 24 ^th Annual Clinical Virology Symposium

Daytona Beach, Florida
April 27-30, 2008

It is common in clinical practice to encounter patient samples that contain mixtures of wild type and resistant HIV viruses. The virco®TYPE HIV-1 v4.2.01 (vT) and the PhenoSense GT™ (PSGT) assays use different methods to account for such mixtures in patient samples undergoing evaluation for antiretroviral drug susceptibility. Karam Mounzer and colleagues from The Jonathan Lax Treatment Center, in collaboration with Cooper University Medical Center and Virco, presented a poster at the 24^th Annual Clinical Virology Symposium showing that even in the presence of mixtures, vT provides quantitative phenotypic information while PSGT reverts to a genotypic interpretation, thereby allowing the vT user to better identify potential residual drug activity in these clinical samples.

This retrospective, single-center study included all patients on a failing antiretroviral drug regimen who had a PSGT assay conducted over a 1 year period between July 1, 2004 and June 30, 2005. Complete mutation/polymorphism lists derived from PSGT assay reports were used to generate corresponding vT reports. A mixture was defined as the presence of wild type amino acid and a resistance associated amino acid substitution at 1 primary or 2 secondary positions of reverse transcriptase and/or protease genome. Among samples containing mixtures, PSGT Net Assessment (NA) results were categorized as susceptible or resistant and vT Resistance Calls (RC) as susceptible, intermediately susceptible, or resistant

The primary objective of the study was to determine the concordance between the PGST Net Assessment and the vT Resistance Call through the following classifications:

• Full concordance: identical susceptible or resistant results with both assays
• Partial concordance: intermediately susceptible result with vT assay and a susceptible or resistant result with PSGT assay.
• Full discordance: opposite susceptible or resistant results with the two assays.

Of the 51 PSGT assays conducted during the 1-year study, 33 (64.7%) contained mixtures, affecting 240 calls per drug in both the NRTI and PI classes. Among 49 calls affected by mixtures and classified as Resistant by PSGT NA (with Phenosense phenotype sensitive, GeneSeq genotype resistant), vT classified 44% and 68% with intermediate susceptibility to NRTIs and PIs respectively, thereby identifying potential residual activity. The authors concluded that the vT has a significant advantage over the PSGT due to its ability to make quantitative assessments of drug susceptibility even in the presence of mixtures.

Data from this new approach provide a conservative assessment of potential resistance based on mutations detected in the viral genotype and eliminates a potential shortcoming of phenotyping by considering the full resistance weight of mutations that may result in sub-optimal responses when present in mixtures.

If you are interested in viewing the complete poster, please click here.

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