The problem

The Proportion of Days Covered (PDC) is a commonly used metric to assess medication adherence using pharmaceutical dispensing or claims data. It is focused on measuring adherence during the implementation phase — i.e., the period between a patient's initiation of medication and discontinuation.

In its simplistic definition, the PDC measures the percentage of days within a specified observation period during which a patient is presumed to have access to their medication (based on having an available supply of dispensed medication). Typically, the numerator is the number of days the patient is covered by medication, and the denominator is the number of days in the observation window (from initiation until discontinuation, or a fixed end date). 

A key strength of PDC is that it avoids double-counting by excluding overlapping days of medication supply from multiple dispensings, which improves its accuracy compared to older measures like the Medication Possession Ratio (MPR).

Despite its widespread use, there is substantial methodological variability in how PDC is calculated and reported across studies and healthcare systems, which can impact the interpretation and comparability of adherence estimates. In patients with cardiovascular disease, the proportion of patients classified as "non-adherent" to their prevention medications can range anywhere between 20 and 60%. 

In our scoping review, we highlighted the variability in methods used to derive the PDC (see Figure below). This underscored the need to develop a new formal reporting framework to standardise the calculation and reporting of PDC to enhance the transparency and comparability of this metric in the literature.

Data source: Dalli LL et al. Br J Clin Pharmacol. 2022 Oct;88(10):4427-4442.