How proportion of days covered is actually calculated, day by day
Proportion of days covered, or PDC, is a key adherence measure in Medicare Part D. Pharmacies see it discussed in quality reports and payer communications, but the math behind it is not always transparent at the counter. PDC is the ratio of days a patient has medication on hand, over a fixed period, for a given drug class. It is measured at the drug class level, not by individual prescription.
The calculation starts by picking a date range, often one full measurement year. Each day in that window, the system checks if the patient has at least one drug in the class available, based on fill dates and days supply. If the patient is covered for a day, it counts as a "covered day." If not, it counts against the total. The numerator is the number of covered days, the denominator is the total number of days in the period.
For example, say a patient starts a statin on January 1 with a 30-day supply. They refill on February 1, again for 30 days. If refills are picked up exactly on time, every day is covered. If there is a gap, say they refill on February 5, then February 2 to 4 are uncovered days. Each gap, no matter how short, brings the PDC down.
Overlapping fills do not double-count. If a patient refills early, extra tablets are carried forward, and the system extends the coverage out, but never counts more than one day per calendar day. Hospitals stays, drug changes, and class switches can complicate the count, but the rule is: covered is covered, uncovered is not.
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The three triple weighted measures: diabetes medications, RASA and statins
Medicare Star Ratings tie pharmacy performance to three specific drug classes for adherence. These are: non-insulin diabetes medications, renin-angiotensin system antagonists (RASAs) and statins. Each class has its own PDC calculation. The Centers for Medicare and Medicaid Services (CMS) counts these three as "triple weighted" when scoring Part D plans, meaning their influence on overall Star Ratings is higher than other measures.
Non-insulin diabetes medications
This group includes oral agents like metformin, sulfonylureas, DPP-4 inhibitors, SGLT2 inhibitors, and similar drugs. Insulin does not count in this measure. Each day the patient has any drug in this class, they are considered covered for diabetes medications.
Renin-angiotensin system antagonists (RASA)
RASAs include ACE inhibitors, ARBs, and certain combination products. These drugs are used for hypertension, heart failure, and kidney protection in diabetes. The measure looks for at least one RASA on hand each day.
Statins
All statin medications are included, whether generic or brand. The PDC method treats simvastatin, atorvastatin, and others as part of the same class. Coverage days count as long as any statin is available. Switching between statins does not interrupt the days covered, but a gap between fills will lower the PDC.
These triple weighted measures matter because they are linked directly to pay-for-performance contracts for plans and sometimes for pharmacies. A high PDC, above a set threshold, is necessary for a plan to receive top scores and related bonuses.
Statin use in persons with diabetes and the related quality measures
Beyond general statin adherence, there is a separate quality measure for statin use in patients with diabetes, ages 40 to 75. The measure looks for at least one statin fill in the measurement year, in patients who are taking diabetes medications and do not have an exclusion.
This measure is different from PDC-based adherence. Here, the pharmacy's role is to identify patients who meet the diabetes medications criteria but are not filling a statin. Plans and networks may prompt the pharmacy to recommend statin therapy, and some may pay a bonus if the pharmacy intervenes and a statin is started.
The two measures work together: PDC for statins counts ongoing adherence, while statin use in diabetes is a one-time check each year. Both measures influence Star Ratings and plan performance payments.
In addition, some value-based contracts with pharmacy networks will tie bonuses to improving the statin use in persons with diabetes rate. Pharmacies that use their workflow and delivery programs to close these gaps can see improved performance metrics.
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Why 80 percent is the cut line and how one late fill moves a patient across it
Eighty percent is the key threshold for PDC adherence. Patients with a PDC of 80 percent or above are counted as adherent for Star Ratings. Below 80 percent, the patient is considered non-adherent. This single cutoff determines whether a plan meets the Star Rating target for adherence measures.
The 80 percent line is not arbitrary. Studies showed that patients with PDC above this level for these drug classes have better health outcomes. As a result, the threshold became the industry standard for plan scoring and bonus payment systems.
How a single late fill can change status
In practice, one missed refill can drop a PDC below 80 percent for the year. Take a patient on a 30-day supply for 12 months. If they refill exactly on time, they reach 360 covered days in a 365-day year, or just under 99 percent. If they are five days late for one refill, those five days are not covered, dropping their PDC to 355 divided by 365, which is about 97 percent. Still above 80 percent, but repeated gaps or a longer delay can push them below the line.
For a patient who already had a few missed days early in the year, a late fill in November or December can be enough to move them from adherent to non-adherent. Conversely, catching up on fills and reducing gaps can bring a patient back over 80 percent before the year ends.
This is why timely refills, delivery, and active patient outreach are tied so closely to pharmacy quality scores. Each day the patient does not have their medication chips away at the PDC, and the margin for error can be small.
The measurement year, the lookback window and when scores lock
The standard measurement year for Star Ratings is January 1 through December 31. Every fill, pickup, and gap within those dates counts toward the PDC calculation. Plans and networks start analyzing adherence as early as the first quarter, but final scores for Star Ratings are based on the entire calendar year.
PDC calculations use a lookback window based on the patient's first fill in the measurement year. If a patient starts therapy mid-year, only the days from their first fill to December 31 are included in the denominator. Early discontinuation or a late start reduces the total days counted, affecting both the numerator and denominator.
Scores "lock" when the measurement year closes and the plan submits its data to CMS. Pharmacies may see monthly or quarterly adherence scores during the year, but these are preliminary. Only the year-end numbers, finalized after CMS audits, are used for official Star Ratings and payment calculations.
Because of this lag, pharmacies sometimes see performance reports that change after the year ends. Adjustments for retroactive claims, new data, or corrections can shift individual patient PDC or the pharmacy's overall adherence rate, but only final locked scores impact contract payments.
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Where pharmacies see their scores and how plans and networks use them
Pharmacies receive PDC and adherence measure scores from multiple sources. Pharmacy services administrative organizations (PSAOs), payer portals, and Star Rating dashboards provide monthly or quarterly updates. These reports usually list each patient's adherence status, gaps in therapy, and opportunities to improve.
Plan sponsors use these scores to manage network performance. Pharmacies with high adherence rates may gain preferred status in networks or be eligible for bonus pools. Pharmacies with low scores can face additional oversight, remediation plans, or even risk exclusion from future contracts.
Networks may break down scores by drug class, by payer, or by prescriber. Some reports show which patients are close to the 80 percent threshold, helping pharmacies target outreach. Refill reminders, med sync, and delivery programs are often prioritized based on these lists.
Plans also use aggregate PDC data to set rates with the Centers for Medicare and Medicaid Services. Higher Star Ratings mean better payments and the ability to market to beneficiaries year-round. Pharmacies that contribute to higher plan scores are considered more valuable to the network.
Independent pharmacies can use their own dispensing software or third-party quality tools to track adherence in real time. Many of these systems now integrate patient engagement, refill calls, and delivery scheduling to support adherence goals.
What med sync and delivery do to the denominator
Medication synchronization, or med sync, is a process where the pharmacy coordinates all of a patient's chronic medications to be refilled on the same day each month. This reduces gaps between fills and makes it easier for patients to stay adherent, especially those on multiple therapies.
By synchronizing fills, med sync limits the uncovered days that drop the PDC. If a patient typically runs out of medication before coming in for their next fill, synchronization can close those gaps. This directly increases the numerator in the PDC calculation, raising the patient's adherence rate.
Local delivery adds another layer of support. When the pharmacy brings medications to the patient's door, especially for those with limited mobility or transportation challenges, it becomes less likely that a fill will be missed or delayed. Every on-time delivery protects a day covered, keeping patients above the 80 percent line.
Signature capture and refill call queues are features that help document on-time delivery and automate the reminders that keep patients on track. By tracking when medications are handed off and following up on missed refills, these workflow tools support adherence goals and provide clear records for audits.
For pharmacies looking to move more patients above the 80 percent threshold, integrated med sync, delivery route building, and documentation tools are practical solutions. Systems that bring these features together let the pharmacy control more steps in the adherence chain, ensuring gaps are addressed before they hurt quality scores.
