Measurement issues in using pharmacy records to calculate adherence to antiretroviral drugs

Researchers often use pharmacy records to calculate adherence to antiretrovirals. Variability in the findings may be due to inconsistent methods of calculating adherence.

Determine the impact on adherence rates of 6 different calculations that include accounting for whether filled antiretroviral prescriptions were picked up and whether the patient had medications at the start of the observation period.

Fifty-six patients of a public care system, who had ordered, but failed to pick up, antiretroviral prescriptions from the clinic pharmacy at least once from September thru December, were identified using an electronic pharmacy database. Their adherence during the 4 months was calculated using refilled doses and picked up doses as a percent of prescribed doses. The effect on the calculation of adherence of medications in the patients' possession from August was examined.

When medications in the patients' possession from August were considered in calculating adherence, the rate was 54% based on the prescription refill date and 33.4% based on the prescription pick-up date. If medications in the patients' possession at the beginning of the observation period were ignored, adherence based on refill was 48.9% and 28.1% based on pick-up. If the start date for calculating adherence was the date of the first refill or pick-up during the first month of the observation period, adherence rates were 56.2% and 41%, respectively.

This study demonstrated that 6 different methods of calculating adherence from pharmacy records yielded adherence rates of 28.1% to 56.2%. Studies using pharmacy records should specify how adherence is calculated.