5.2 Proportion of Days Covered (PDC)
Key Takeaways
- PDC = (number of days covered in the measurement period) ÷ (number of days in the measurement period), usually expressed as a percent.
- Days covered come from dispensing claims and days’ supply; overlapping early refills are stacked forward so coverage is not double-counted on the same calendar days.
- PDC reflects possession (fill) coverage, not confirmed ingestion—cash fills elsewhere and samples can understate true use.
- Worked numeric examples show how late refills and gaps pull PDC below the common 80% adherence threshold used in quality programs.
- Technicians compute and present PDC inputs/flags; pharmacists use results for CMR/TMR interventions and clinical interpretation.
5.2 Proportion of Days Covered (PDC)
Quick Answer: PDC = days covered ÷ days in the measurement period. If a patient has 144 covered days in a 180-day window, PDC = 144/180 = 0.80 (80%). Overlapping early refills are stacked forward so the same calendar days are not counted twice. PDC is a possession metric from claims—not proof the patient swallowed every dose.
After you can calculate days’ supply, Proportion of Days Covered (PDC) is the standard adherence metric used in many quality programs, including Pharmacy Quality Alliance (PQA) measures that feed Medicare Part D Star Ratings. MTM technicians support gap lists by understanding how PDC is built, where gaps appear, and which arithmetic mistakes falsely raise or lower the score.
Definition and Formula Intuition
Imagine a measurement period as a row of calendar days (for example, 180 days). Each day the patient theoretically has medication available from a valid fill’s days’ supply counts as covered. Days without remaining supply from prior fills are uncovered (gaps).
| Component | Meaning |
|---|---|
| Measurement period | Fixed window (often aligned to plan year or a rolling lookback) |
| Days covered | Distinct calendar days with medication available based on fills/DS |
| PDC | Days covered ÷ days in period |
| PDC % | PDC × 100 |
Formula: PDC = (Days Covered) / (Days in Measurement Period)
Intuition: PDC asks, “For what fraction of the window did the patient have the drug on hand according to pharmacy claims?” It does not ask whether every dose was taken, whether the patient understood directions, or whether clinical outcomes improved—those require CMR conversation and pharmacist judgment.
Hospitalization adjustments and specialty measure nuances exist in full PQA specifications; for this certificate’s calculation focus, master basic PDC from fills, days’ supply, gaps, and overlap stacking. You are not expected to memorize every exclusion rule for inpatient stays beyond knowing that published measure specs may adjust denominators in real programs.
Worked Example 1 — Simple non-overlapping fills
Period: 180 days. Drug: atorvastatin, 30-day supplies. Fills: four fills, no overlap (exactly end-to-end would be continuous; here assume four separate 30-day covers with gaps between some fills).
If the patient filled four times × 30 days = 120 days of supply and none of those covered days fall outside the period or overlap each other inside the period, days covered ≈ 120.
PDC = 120 / 180 = 0.667 → 66.7% (about 67%).
This is below the common 80% adherence threshold used by PQA Star adherence measures (next section). The technician flags the patient for pharmacist adherence outreach or TMR.
Worked Example 2 — Meeting the 80% bar
Period: 180 days. Target days covered for ≥80%: 0.80 × 180 = 144 days.
Scenario: five fills of 30-day supply with minimal gap and no harmful double-counting → up to 150 days of supply available.
If stacking/overlap adjustment still leaves 150 distinct covered days inside the 180-day window:
PDC = 150 / 180 = 0.833 → 83.3% → meets ≥80%.
If instead only 140 distinct covered days remain after accounting for late starts:
PDC = 140 / 180 = 0.778 → 77.8% → fails the 80% threshold even though five fills occurred—because timing left gaps.
Teaching point: Fill count alone is not PDC. Timing and uncovered days matter.
Worked Example 3 — Full-year window
Period: 365 days. Fills: 10 × 30-day supplies = 300 days of dispensed supply, non-overlapping and fully inside the year.
PDC = 300 / 365 ≈ 0.822 → 82.2% (passes 80%).
If the patient only filled 8 × 30 = 240 days:
PDC = 240 / 365 ≈ 0.658 → 65.8% (fails).
Reverse planning for technicians: Days needed for 80% in a 365-day year ≈ 0.80 × 365 = 292 days of coverage. Roughly ten 30-day fills (300 days) can clear the bar if gaps are small; nine 30-day fills (270 days) max out at 270/365 ≈ 74% even with perfect end-to-end use—so a patient on 30-day retail fills typically needs about ten fills/year of continuous coverage, or fewer fills if using 90-day supplies.
Worked Example 4 — 90-day fills
Period: 365 days. Patient receives four mail-order 90-day fills = 360 days of supply.
If nearly all covered days fall in-period with only tiny uncovered edges: PDC ≈ 360/365 ≈ 98.6%.
If the patient delays the second fill and ends with only 300 distinct covered days: PDC = 300/365 ≈ 82.2% (still passes) versus 270 covered days → 270/365 ≈ 74.0% (fails).
90-day fills improve convenience but late mail-order still creates large gap blocks that crush PDC quickly.
Overlap: Why Early Refills Do Not Inflate PDC Freely
Suppose a 30-day fill on Day 1 and another 30-day fill on Day 25 (5 days early).
Incorrect approach: Count 30 + 30 = 60 covered days inside a short window that only has ~55 calendar days—double-counting the overlap.
PQA-aligned intuition: Stack the new supply to begin when the previous supply would have been exhausted (push coverage forward). The early refill extends how far into the future the patient is covered; it does not grant two covered credits for the same calendar day.
| Approach | Effect on PDC |
|---|---|
| Double-count overlap days | Artificially inflates PDC |
| Stack / push remaining supply forward | Credits extra days after prior DS ends |
| Ignore early refill entirely | May understate future coverage |
Exam items often ask which method matches quality-measure thinking: do not double-count overlapping days; carry remaining supply forward per measure logic.
Common PDC Traps for Technicians
- Using quantity without days’ supply math — wrong DS upstream → wrong covered days.
- Confusing PDC with MPR (Medication Possession Ratio) — related possession ideas exist; PDC emphasizes distinct covered days with overlap handling. Stick to the PDC definition above unless an item explicitly names another metric.
- Assuming claims equal ingestion — PDC cannot see discarded tablets, shared meds, or doses skipped while bottles remain.
- Missing out-of-network cash fills — claims-based PDC can look worse than reality if the patient paid cash elsewhere.
- Acute meds in chronic windows — antibiotics should not drive chronic adherence measures; measure populations are filtered in real PQA specs.
- Denominator mistakes — PDC uses days in the measurement period (and measure-specific enrollment rules in full specs), not “days since first fill” unless the specification defines the index date that way.
Realistic MTM Workflow Use
A technician pulls a Star adherence gap list showing Ms. Rivera’s statin PDC at 72% in a year-to-date window. The tech verifies fill dates, recalculates approximate covered days from DS, confirms late 30-day fills created ~100 uncovered days, and prepares a summary for the pharmacist before a TMR call. The technician does not tell the patient to “double up doses” to catch up, and does not change the statin strength.
Basic PDC fluency turns gap reports from mysterious percentages into explainable timelines—exactly what Domain 2 calculation items reward.
A patient has 120 days covered in a 180-day measurement period. What is the PDC, and does it meet an 80% threshold?
In a 365-day measurement period, approximately how many covered days are required to reach at least 80% PDC?
A patient refills a 30-day supply 5 days early. Which approach aligns with PQA-style PDC overlap handling?
Which statement correctly describes a limitation of claims-based PDC?