Using different calculations of pharmacy refill adherence to predict virological failure among HIV-infected patients

Refill data are increasingly used to assess adherence in HIV-infected patients on combination antiretroviral therapy. However, it is not clear how feasible this method is when multiple pharmacies are involved. Also, the effects of inclusion of leftover medication from previous refills and prescribed treatment time on adherence calculations are unknown. We addressed these questions in the present study. Adult HIV-1-infected patients were recruited at the outpatient clinic of the Academic Medical Centre in Amsterdam and asked for their pharmacies' names. Refill data were obtained from pharmacies. Percentages of patients misclassified as nonadherent when disregarding leftover medication and prescribed treatment interruptions were calculated. Finally, we investigated whether an average adherence calculation of all drugs or a calculation based on one drug in the regimen best predicted virological failure (plasma HIV-1 RNA >40 copies/mL). Two hundred one patients were included. Collecting data from multiple pharmacies (132) was found to be feasible. Forty-three percent of patients were misclassified as nonadherent when disregarding leftover medication and 2 percent when disregarding prescribed treatment time. There was no difference in predicting virological failure by different calculations of adherence. These findings suggest that studies using pharmacy refill data should include leftover medication