Measuring Medication Adherence Using PDC

The primary outcome was change in adherence in the presumed parent of each eligible child. Adherence was assessed by applying the proportion of days covered (PDC) metric to pharmacy claims data, a standard quality measure.^{27 (link),28 (link),29 (link)} To measure PDC, we generated a supply diary for the parent’s medication by aggregating consecutive fills based on dispensing date and days supplied, starting 539 days before the child’s index date (or the parent’s first fill before that date) through 360 days after the index date.^{11 (link)} We allowed supply to accumulate to 180 days; days spent in a facility were considered covered days. Parents were censored on loss of enrollment.
Using this supply diary, we calculated PDC within each month (normalized to 30 days) from 360 days before the index date to 360 days after (24 months total). Overall, PDC was calculated by dividing the number of days with medication available each month by the number of days the parent contributed that month. We measured PDC as both a continuous (primary outcome) and secondary dichotomous adherence measure, with PDC of 80% or greater considered optimal.^{27 (link),30 (link),31 (link)}