ObjectiveTo compare self-reported nonadherence with antiretroviral therapy (ART) with predose plasma levels of protease inhibitor (PI).DesignA cross-sectional study of consecutive patients from a university-based HIV clinic in Rome, Italy, was conducted. One hundred and forty HIV-infected patients were prescribed regimens containing ritonavir or indinavir. A patient questionnaire assessing knowledge of treatment regimen, adherence behavior, reasons for taking and missing therapy, factors influencing adherence, and health behaviors was administered. A predose PI plasma level was measured concurrently.ResultsBy patient report, 12% missed at least one dose “yesterday,” and 24% missed doses in the last 3 days; 14% had a predose plasma concentration below the assay limit of quantitation (2 ritonavir and 18 indinavir samples). Confusion, poor psychological well-being, long office wait, running out of drugs between visits, having relatives to remind the patient to take medication, children, and alteration of sense of taste were related to unquantifiable predose levels of PI. In multivariable analysis, reported nonadherence (odds ratio [OR], 15.8; 95% confidence interval [CI], 4.0–63.3) and confusion (OR, 9.9; 95% CI, 1.4–69.6) were related to unquantifiable predose levels of PI.ConclusionSelf-report of missing a dose of antiretroviral medication “yesterday” was related to an unmeasurable plasma PI level.