Medication Adherence Metrics

The two most widely used approaches for calculating medication adherence are medication possession ratio (MPR) and proportion of days covered (PDC).

The MPR is likely to overestimate adherence levels because the formula does not account for early refills. Using MPR to measure adherence can produce outcomes greater than 100. Consider if a patient were to pick up their prescription for 30 tablets of atorvastatin 40mg taken once daily and came back to the pharmacy 25 days later to refill this prescription. In this case, the patient would be five days early picking up their atorvastatin refill. The sum of days’ supply for all fills in the refill period is 30 while the number of days in the refill period would be 25. MPR would be 1.2. This problem was resolved by the PDC adherence measurements.


PDC caps the adherence rate at 100 because the PDC accounts for patients that refill their prescriptions early. The overlapping days’ supply on an early refill will be moved forward to the first day that the patient would not have medication from their previous prescription fill. PDC has been endorsed by Pharmacy Quality Alliance (PQA). The Centers for Medicare and Medicaid Services (CMS) have incorporated PDC as the medication adherence metric for plan ratings. PHSI recommends use of the PDC when calculating patient adherence.


Published June 2018

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