Background: Combination antiretroviral therapy with protease inhibitors has transformed HIV infection from a terminal condition into one that is manageable. However, the complexity of regimens makes adherence to therapy difficult.

Objective: To assess the effects of different levels of adherence to therapy on virologic, immunologic, and clinical outcome; to determine modifiable conditions associated with suboptimal adherence; and to determine how well clinicians predict patient adherence.

Design: Prospective, observational study.

Setting: HIV clinics in a Veterans Affairs medical center and a university medical center.

Patients: 99 HIV-infected patients who were prescribed a protease inhibitor and who neither used a medication organizer nor received their medications in an observed setting (such as a jail or nursing home).

Measurements: Adherence was measured by using a microelectronic monitoring system. The adherence rate was calculated as the number of doses taken divided by the number prescribed. Patients were followed for a median of 6 months (range, 3 to 15 months).

Results: During the study period, 45,397 doses of protease inhibitor were monitored in 81 evaluable patients. Adherence was significantly associated with successful virologic outcome (P < 0.001) and increase in CD4 lymphocyte count (P = 0.006). Virologic failure was documented in 22% of patients with adherence of 95% or greater, 61% of those with 80% to 94.9% adherence, and 80% of those with less than 80% adherence. Patients with adherence of 95% or greater had fewer days in the hospital (2.6 days per 1000 days of follow-up) than those with less than 95% adherence (12.9 days per 1000 days of follow-up; P = 0.001). No opportunistic infections or deaths occurred in patients with 95% or greater adherence. Active psychiatric illness was an independent risk factor for adherence less than 95% (P = 0.04). Physicians predicted adherence incorrectly for 41% of patients, and clinic nurses predicted it incorrectly for 30% of patients.

Conclusions: Adherence to protease inhibitor therapy of 95% or greater optimized virologic outcome for patients with HIV infection. Diagnosis and treatment of psychiatric illness should be further investigated as a means to improve adherence to therapy.