• Poster 788: Potential Clinical Impact of Small Differences between Virco Antivirogram and ViroLogic PhenoSense Assays for Abacavir in 3TC Experienced Patients (Authored by E.R. Lanier, T. Melby, M.H. St. Clair, D. Thorborn, G. Pearce, S. Hetherington, L. Smiley, and S. Lafon. Glaxo Wellcome Res. Triangle Park, NC, and Greenford, UK)
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Poster 788 examined some of the performance differences between the two phenotypic assays (Virco, Antivirogram and Virologic, PhenoSense) for antiretroviral resistance. Plasma HIV samples were examined from patients who were ZDV/3TC experienced, and in whom abacavir (ABC) was added to their regimen (CNA2003 study). Specifically, the question of ABC resistance was examined, and how comparable the two assays were in delineating ABC resistance. Samples were also sequenced to determine the genotype and whether the codon change for 3TC resistance (M184V) was present. In addition, since the M184V mutation is one of several associated with ABC resistance, what were the viral load responses after the addition of ABC in the presence of a m184v mutation and 3TC phenotypic resistance. 14 and 19 samples were analyzed for ABC susceptibility with the PhenoSense and Antivirogram assays respectively. When the M184V mutation was present, results showed a 3.3-fold decrease in ABC susceptibility with the PhenoSense test and a two-fold decrease with the Anitvirogram. These results would be interpreted as decreased sensitivity in the Virologic and sensitive in the Virco assay. There was not a significant difference in viral load response to the addition of ABC in those patients that had a M184V mutation. These results indicate that clinically, a M184V mutation or low levels of decreased susceptibility to ABC, does not appear to affect a viral load response when ABC is added to a regimen. However, the phenotypic assays scored the significance of the fold decrease in susceptibility differently. Thus, depending on the assays utilized, clinicians could be given results with different interpretations and thus guide treatment differently. Clinicians should be aware that the two phenotypic assays are not identical in their methodologies and therefore may yield different results. More comparative studies like this one are needed to understand the performance and interpretation of phenotypic results.