The detection of non-adherence by self-administered questionnaires can be optimized by protease inhibitor plasma concentration determination.

Self-reported adherence and plasma protease inhibitor concentration (PPIC) were assessed in 642 HIV-infected patients at month 4 of a single protease inhibitor-containing regimen. PPIC was below the limit of quantification (LOQ) in 32% of non-adherent patients and in 8% of adherent patients. The relationship between non-adherence and a detectable HIV viral load was enhanced when adherent patients with PPIC below LOQ were considered to be non-adherent. PPIC combined with self-report may more reliably detect non-adherence.