What medication adherence is
Medication adherence is the extent to which a patient takes a medication as prescribed: the right drug, at the right dose and timing, sustained over time. It is different from simply filling a prescription. A patient can fill a script and still be non-adherent by skipping doses or stopping early. The World Health Organization estimates that adherence to long-term therapy for chronic disease averages only about 50% in developed countries, which is why it is one of the most consequential and overlooked problems in chronic care.
How it is measured: PDC
The industry-standard measure is Proportion of Days Covered (PDC), the share of days in a measurement period on which the patient had the medication available, calculated from pharmacy fill and supply data. A PDC of 80% or higher counts the patient as adherent. PDC has largely replaced the older Medication Possession Ratio, which can exceed 100% when patients refill early and so overstates true adherence. The 80% threshold is stewarded by the Pharmacy Quality Alliance (PQA) and adopted by CMS.
Why it matters in Medicare
Three medication-adherence measures drive a large share of a Part D plan's quality score, and all three are triple-weighted: adherence for diabetes medications, for hypertension treated with renin-angiotensin system antagonists, and for cholesterol treated with statins. Because triple-weighted measures count three times as much as a standard process measure, these three dominate the Part D summary score, and a fraction of a star can be worth millions in quality bonus payments. Beyond Star Ratings, non-adherence drives avoidable hospitalizations and worse outcomes, at an estimated cost to the U.S. health system of $100 to $300 billion a year.
The 80% cliff, and why timing decides the year
PDC is annualized, so days lost early in the year cannot be bought back later. A member who runs out of medication in February and refills in May has already burned roughly 90 covered days that no late refill can recover. The threshold is also a hard cliff: a member at 79% is scored non-adherent and a member at 81% is adherent, with no partial credit between them. The members who matter most are those hovering near the line, where one or two saved refills flips both the member and the rate. Outreach in the fourth quarter, aimed at members already mathematically below 80%, produces no measure movement.
How to improve adherence
Effective strategies include simplifying regimens, synchronizing and 90-day fills, removing cost and access barriers, and timely outreach the moment a refill gap appears. The highest-leverage move is closing the loop quickly: detect the refill gap, reach the patient or prescriber, resolve the barrier, and confirm the medication is actually back in hand. A reminder call does not change PDC. Only a confirmed fill that adds covered days does.
How Pelica holds the line
Pelica's Pharmacy and Part D copilot tracks PDC at the member and drug-class level in real time, flags members trending toward the 80% cliff while days remain to recover, and prioritizes outreach by recoverability rather than raw gap size. Voice AI calls the pharmacy and the prescriber, follows up, and escalates to a human only when one is truly needed. Across Pelica deployments, customers hold 96% medication adherence on the three triple-weighted Part D measures.
Related terms
See PDC for the calculation, triple-weighted measures for why these three dominate the score, MTM for the medication-review program that supports adherence, and the three Part D adherence measures for how each is scored.