Phenotypic drug susceptibility testing predicts long-term virologic suppression better than treatment history in patients with human immunodeficiencyvirus infection

To assess the value of phenotypic drug susceptibility testing as a predicto r of antiretroviral treatment response in human immunodeficiency virus (HIV )-infected people, drug susceptibility testing was performed retrospectivel y on plasma samples collected at baseline in a cohort of 86 antiretroviral- experienced, HIV-infected people experiencing treatment failure and initiat ing a new antiretroviral treatment regimen. Two separate criteria for reduc ed drug susceptibility were evaluated. In multivariate analyses, phenotypic susceptibility was an independent predictor of time to treatment failure ( adjusted hazards ratio [HR], 0.70; 95% confidence interval [CI], 0.55-0.90; and adjusted HR, 0.76; 95% CI, 0.61-0.95, with reduced drug susceptibility cutoffs defined as 4.0-fold and 2.5-fold higher than reference virus IC50 values, respectively). Previous protease inhibitor experience was also a si gnificant independent predictor. Notably, drug susceptibility predicted on the basis of treatment history alone was not predictive of time to treatmen t failure. In this cohort, phenotypic testing results enhanced the ability to predict sustained long-term suppression of virus load.