Proportion of days covered (PDC) estimates medication adherence by looking at the proportion (or percentage) of days in which a patient has access to the medication, over a defined measurement period. It is based on the script dispensing data and days' supply for each dispensing (how long the medicine should last before resupply is expected).
Typically, a patient is considered as adherent if the PDC is greater than 80% for an individual (an adherence threshold of 0.8).
For example, using a measurement period of three months, in "month 1-3", patients are expected to use the medicine for the entire three months (90 days). Assume a patient had three scripts within this period and each script has a 30 day supply, but the patient was 10 days late when refilling the second script (after the previous supply had run out) and 5 days late when refilling the third script. This patient's medicine covers 75 days (15 days are uncovered) in "month 1-3", and adherence (PDC) will be 83.3% (75/90 x 100% = 83.3%).
Note:
- The PDC method assumes patient's take the medicine as prescribed.