Which adherence metric is typically preferred to measure whether patients have medication available across days?

• A. Proportion of Days Covered (PDC)
• B. Medication Error Rate
• C. Average Days Supply per fill
• D. Pharmacy Turnover Rate

Answer: (A)

When evaluating whether patients have medication available across days, you want a metric that reflects continuous supply over a period. Proportion of Days Covered does this by using pharmacy fill data and days supplied to determine how many days within the measurement period the patient actually had medication on hand, then dividing by the total number of days in that period. The result, capped at 100%, gives a clear view of true day-to-day availability, not just possession at a single point in time. For example, if a patient had meds on hand for 290 of 365 days, the PDC would be 290/365, about 79.5%. Early refills can inflate total days supplied, but the PDC remains capped at 100%, preventing overestimation of adherence.

Medication Error Rate focuses on safety events rather than whether the patient consistently had the medication available, so it’s not about continuity of supply. Average Days Supply per Fill describes how long each refill lasts but doesn’t account for gaps between fills or overall continuity across the period. Pharmacy Turnover Rate is an inventory metric tied to store operations and stock levels, not whether patients actually had medication available across days.