Why these three measures are different
Medicare Part D has roughly 14 measures inside the Star Ratings, but three pull disproportionate weight:
- Diabetes Medications (PDC-DM)
- Renin-Angiotensin System Antagonists (PDC-RASA), covering ACE inhibitors, ARBs, and direct renin inhibitors
- Statins (PDC-STA)
Each of these is triple-weighted in CMS's overall Star Rating calculation. A typical Part D measure carries a weight of 1; these carry a weight of 3. Plans that win three points on these measures and lose three points elsewhere will net positive on Stars.
The triple weighting is not arbitrary. CMS prioritizes adherence measures because chronic-disease adherence is one of the highest-leverage drivers of total cost of care for an MA population.
What PDC actually measures
Proportion of Days Covered (PDC) ≥ 80% is the threshold. The denominator is the number of eligible members who received at least two fills in the measurement year. The numerator is members whose pill supply, summed across fills, covers at least 80% of the days from their first fill through year-end.
A member who fills a 90-day prescription in January and again in April but skips a refill in July will likely fall below 80% PDC for the year, regardless of why. CMS does not adjust for clinical context, dose changes, or hospitalization unrelated to the medication.
The cliff effect
Two patients can have nearly identical adherence behavior and produce wildly different PDC outcomes:
- Patient A: 80.0% PDC. Counted as adherent.
- Patient B: 79.9% PDC. Counted as non-adherent.
The 80% cut is binary at the member level. But at the contract level, the percentage of members above 80% is what gets reported. CMS then maps that percentage to a star score using cut points published each fall.
For PDC-DM in a representative reporting year, the approximate cut points were:
| Star score | Approximate share of members adherent |
|---|---|
| 2 stars | 72% to 80% |
| 3 stars | 80% to 86% |
| 4 stars | 86% to 91% |
| 5 stars | Above 91% |
The same 5-percentage-point swing, say from 84% to 89% adherent members, moves a contract from solidly 3 stars to solidly 4 stars on this measure. With triple weighting, that single shift is equivalent to moving three single-weighted measures by a full star each.
Why $13 billion is on the table
CMS pays Quality Bonus Payments to MA contracts rated 4 stars or higher. The total annual pool exceeds $13 billion. As of 2026, only about 40% of MA contracts hit 4 stars or higher, down from peaks above 50% in earlier years after CMS tightened the cut-point methodology and removed COVID-era flexibilities.
For a contract on the bubble, moving the three triple-weighted PDC measures from 3 stars to 4 stars is often the difference between qualifying for QBP and not. On a 50,000-life MA contract, that gap is typically worth $25 million to $40 million per year in payment, before accounting for rebate reinvestment.
The operational levers that work
Most plans treat adherence as a member-engagement problem. It is partially a data and timing problem.
- Refill timing visibility. A plan that knows, on day 21 of a 30-day fill, that a member has not refilled, can intervene before the gap opens. Most plans learn about gaps from PDE data 60 to 90 days after the fact.
- Dose synchronization. Aligning all of a member's chronic medications to a single fill date raises PDC by reducing the number of trips to the pharmacy, where each trip is an opportunity to skip.
- Targeted MTM (Medication Therapy Management). MTM is a CMS-required intervention but historically used as a check-the-box CMR call. High-performing plans target MTM at members in the 65% to 80% PDC band, the population most movable.
- Prescriber outreach for new starts. Roughly one-third of statin non-adherence is driven by patients who never refill the first prescription. Engaging the prescribing physician within 30 days of the first fill closes a meaningful share of these.
- Removing operational friction. Mail-order conversion, 90-day fills, and auto-refill enrollment all push baseline PDC up by 4 to 8 percentage points without any member behavior change.
Where plans leave money on the table
Three failure modes are common:
- Treating adherence as the pharmacy team's problem. Adherence depends on the prescribing physician (start), the pharmacy (fill), and the care team (continuation). Plans that do not coordinate across all three will hit a ceiling around 82% PDC.
- Acting on stale PDE data. Most plans rely on monthly Prescription Drug Event reconciliation files. By the time a gap is visible in PDE, the member is already 30 to 60 days non-adherent. Real-time fill data via E1 transactions or pharmacy partnerships closes that gap.
- Generic CMR scripts in MTM. Comprehensive Medication Reviews that read from a script lift adherence by close to zero. CMRs that surface the specific gap, in the specific medication, with the specific clinical reason, lift adherence by 8 to 15 percentage points in published studies.
"PDC is won or lost in the 21-to-45-day window after a missed refill. Plans that act in that window beat plans that don't, and it has almost nothing to do with the call script."
Sources and further reading
- CMS: Medicare Part C and D Performance Data and Star Ratings Technical Notes (annual, October release)
- Pharmacy Quality Alliance (PQA): endorsed PDC measure specifications
- CMS: Medicare 2026 Part C and D Star Ratings Data (released October 2025)