ObjectiveTo determine the ability of intermediate plasma viral load (pVL) measurements to predict virologic outcome at 52 weeks of follow-up in clinical trials of antiretroviral therapy.MethodsIndividual patient data from three clinical trials (INCAS, AVANTI-2 and AVANTI-3) were combined into a single database. Virologic success was defined to be plasma viral load (pVL) <500 copies/ml at week 52. The sensitivity and specificity of intermediate pVL measurements below the limit of detection, 100, 500, 1000, and 5000 copies/ml to predict virologic success were calculated.ResultsThe sensitivity, specificity, and positive and negative predictive values of a pVL measurement <1000 copies/ml at week 16 to predict virologic outcome at week 52 were 74%, 74%, 48%, and 90%, respectively, for patients on double therapy. For patients on triple therapy, the sensitivity, specificity, and positive and negative predictive values of a pVL measurement <50 copies/ml at week 16 to predict virologic outcome were 68%, 68%, 80%, and 47%, respectively.ConclusionsFor patients receiving double therapy, a poor virologic result at an intermediate week of follow-up is a strong indicator of virologic failure at 52 weeks whereas intermediate virologic success is no guarantee of success at 1 year. For patients on triple therapy, disappointing intermediate results do not preclude virologic success at 1 year and intermediate successes are more likely to be sustained.