PDC: Proportion of Days Covered, Explained | Pelica Health

Short answer

PDC, or Proportion of Days Covered, is the fraction of days in a measurement period that a member had a medication available, based on fill and supply data. A PDC of 80% or higher counts a member as adherent. It is the threshold used by the three triple-weighted Part D adherence Star Ratings measures.

What PDC is

PDC stands for Proportion of Days Covered. It is the fraction of days in a measurement period that a member had a given medication available to take, calculated from pharmacy fill dates and days supply. A PDC of 80% or higher counts the member as adherent for that drug class.

The metric is used to score medication adherence in Part D, the prescription drug benefit of Medicare. PDC is the adherence definition the Pharmacy Quality Alliance (PQA) developed and the Centers for Medicare and Medicaid Services (CMS) adopted for the adherence measures in Star Ratings, the quality rating system CMS applies to Medicare Advantage and Part D plans.

Why PDC matters

PDC drives a large share of a Part D plan's quality score, and that score drives revenue. Three adherence measures use the 80% PDC threshold, and all three are triple-weighted. Because triple-weighted measures count three times as much as a standard process measure, these three dominate the Part D summary score.

The 80% threshold is a per-member cliff. A member at 79% is scored as non-adherent and a member at 81% is scored as adherent, and the two are scored identically within their bucket: there is no partial credit for the 79% member and no extra credit for a member at 95%. A handful of members crossing the line, in either direction, can move the measure rate enough to change a plan's star.

The timing is unforgiving. PDC counts covered days across the whole period, so days lost early 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.

How PDC is calculated

PDC is the number of days in the measurement period that the member had at least one drug in the class on hand, divided by the number of days the member was eligible during that period. The numerator is built from fill dates and days supply on pharmacy claims, with overlapping supply from early refills carried forward rather than double-counted.

80%

PDC threshold at or above which a member counts as adherent

3x

Weight CMS assigns to each of the three Part D adherence measures

3

Adherence measures on PDC: diabetes meds, RAS antagonists, statins

The 80% cutoff applies to three drug classes scored separately: diabetes medications, hypertension medications in the RAS antagonist class (ACE inhibitors and angiotensin receptor blockers), and cholesterol medications in the statin class. A member is included in a measure once they have two or more fills in the class, and is then scored adherent or non-adherent on the 80% line.

Because the three measures are triple-weighted, they carry far more weight in the Part D summary score than the count of measures suggests. A small absolute gain in the adherent rate on any of the three can move the summary rating, which is why teams track PDC at the member level and intervene before a member crosses below the line.

Common mistakes teams make with PDC

• Acting too late in the year. Outreach in the fourth quarter targets members who are already mathematically below 80% for the year. The covered days they lost in the first half cannot be recovered, so the spend produces no measure movement.
• Prioritizing by gap size instead of recoverability. A member at 60% who cannot mathematically reach 80% is a worse use of effort than a member at 78% who can still cross the line. The cliff means recoverability, not the size of the shortfall, should set priority.
• Treating the three measures as one. Diabetes, RAS antagonists, and statins are scored separately. A member can be adherent on statins and non-adherent on diabetes medication, and each class needs its own tracking.
• Ignoring early-refill and overlap rules. Stockpiling from early refills does not push PDC above 100%, and supply on hand at the end of the period does not carry credit forward. Teams that count fills rather than covered days misread who is actually at risk.
• Outreach with no fill confirmation. A reminder call does not change PDC. Only a confirmed fill that adds covered days does, so interventions have to close the loop to the pharmacy claim.

How Pelica handles PDC

Pelica's Pharmacy and Part D Copilot tracks PDC at the member and drug-class level in real time, flags members who are trending toward the 80% cliff while days remain to recover, and prioritizes outreach by recoverability rather than raw gap size. Across Pelica deployments, customers hold 96% medication adherence on the three triple-weighted Part D measures.

Related terms

PDC sits inside the broader Part D and quality framework. See Part D for the prescription drug benefit that these measures score, MTM (Medication Therapy Management) for the adherence interventions that support PDC, and Star Ratings for how the triple-weighted adherence measures roll into a plan's overall rating.

Sources

• Pharmacy Quality Alliance (PQA): adherence measure specifications, including PDC
• CMS: Medicare Advantage rates, statistics, and risk adjustment

Frequently asked questions

Common questions about Proportion of Days Covered and the Part D adherence measures.

What PDC counts as adherent?

A PDC of 80% or higher counts a member as adherent. The threshold is a hard cliff: a member at 79% is scored as non-adherent and a member at 81% is scored as adherent, with no partial credit between them.

How is PDC calculated?

PDC is the number of days in the measurement period that a member had at least one drug in the class on hand, divided by the number of days the member was eligible in that period, based on fill dates and days supply from pharmacy claims.

Which Part D measures use PDC?

Three adherence measures use the 80% PDC threshold: diabetes medications, hypertension treated with RAS antagonists (ACE inhibitors and ARBs), and cholesterol treated with statins. All three are triple-weighted in Part D Star Ratings.

Why does the 80% PDC threshold matter so much?

The three PDC adherence measures are triple-weighted, so they dominate the Part D summary score. Because the 80% threshold is per-member and binary, a small number of members crossing the line moves the measure rate and the overall rating.

Can a late refill recover a member who fell below 80% PDC?

Often not fully. PDC counts covered days across the whole measurement period, so days already lost earlier in the year cannot be recovered by refilling late. The earlier a gap is closed, the more days remain to stay above the line.

Hold the 80% line on every triple-weighted measure.

Pelica's Pharmacy and Part D Copilot tracks PDC per member and per drug class, flags the members still recoverable before the cliff, and prioritizes outreach by days remaining. Across Pelica deployments, customers hold 96% medication adherence on the three triple-weighted Part D measures.