Impact of including physician’s prescribing directions on calculations of medication possession ratios

Medication adherence studies typically use pharmacy-dispensing data to infer drug exposures. These studies often require calculations reflecting the intensity and duration of drug exposure. The typical approach to estimating duration of drug exposure is to use dispensing dates and day supply. Often, pharmacy databases have random and/or systematic errors causing improbable calculations. These errors become particularly problematic when estimating medication duration in drugs with complicated dosing schedules. Experts recommending cleaning data or removing erroneous data before analysis, but do not provide instructional guidelines. We developed an algorithmic approach to improve estimation of drug-course duration, dosing and medication possession ratios (MPRs). This study compares estimated MPRs produced by the standard method with MPRs by the algorithmic approach. Methotrexate was chosen as the first drug to implement the algorithm because of its widespread use for rheumatoid arthritis (RA) and for its complexity in dosing schedules.