Signature
a_i = max(f_i, a_prev + S_prev)
| Inputs | Definition | Unit |
|---|---|---|
f_i | Day on which fill i was dispensed | day number on the same scale as a_i |
a_prev | Adjusted start day of the previous fill of the same drug, itself already shifted | day number |
S_prev | Days' supply of the previous fill of the same drug, so that its supply ends on day a_prev + S_prev minus 1 | days |
a_i | Adjusted start day of fill i of the drug, after shifting | day number, with the index date as day 1 |
|---|
Function
Proportion of days covered from pharmacy fill dates and days' supply function
Maps a patient's dispensing records, each a fill date and a days' supply, and an observation period to the proportion of days covered (PDC), the share of days in the period on which at least one fill provides medicine, each day counted once. Same-drug refills collected before the previous supply runs out are shifted to start the day after it ends, overlaps between different drugs in a class are not shifted, supply running past the end of the period is ignored, and the CMS method removes inpatient and skilled nursing facility stay days. The records follow the notation of the Proportion of Days Covered article. The medication possession ratio, which sums days' supply instead, is HE-FM-MPR-001.
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Implementations
Excel
Shifted start day from the dispensing day and the previous fill
With fills sorted by dispensing day, Excel takes the later of the named cell holding this fill's dispensing day and the previous fill's shifted start plus its days' supply. In a fill table the formula is filled down a helper column, with the first fill's start equal to its dispensing day.
=MAX(FillDay,PrevStart+PrevSupply)
Assumptions
Earlier supply finished before a refill is started
A patient who refills early finishes the supply from the preceding fill before starting the new one, so the early refill extends coverage instead of duplicating it. The rule cannot tell stockpiling for later use from doses already missed.
Shift applied to the same drug only, not across a class
The shift applies to fills of the same drug. Where a measure covers a class, overlapping fills of different drugs are treated as taken together and are not shifted, and a day counts once if at least one drug is available. In Example 3 (Tables L-4 and L-5) of Attachment L of the CMS technical notes, lisinopril overlapping captopril is left unadjusted.
Worked examples
Second fill dispensed on day 22 shifted to day 31
Fill 2 of the article's history is dispensed on day 22 with 9 days of fill 1 left, so it starts on day 31, the day after fill 1 (days 1 to 30) ends.
f_i = 22; a_prev = 1; S_prev = 30; a_i = 31
Fourth fill after a gap keeps its dispensing day
Fill 4 is dispensed on day 121, after shifted fill 3 (days 61 to 90) has ended, so it starts on day 121 and days 91 to 120 stay uncovered.
f_i = 121; a_prev = 61; S_prev = 30; a_i = 121
Fifth fill shifted beyond the end of the 180-day period
Fill 5 is dispensed on day 170 while the 90-day fill 4 lasts to day 210, so it starts on day 211, after the period has ended, and adds no covered days.
f_i = 170; a_prev = 121; S_prev = 90; a_i = 211
Common errors
Shifting from the unshifted end of the previous fill
Comparing fill 3 (day 45) with the original end of fill 2 (day 51) instead of its shifted end (day 60) starts fill 3 on day 52, so days 52 to 60 are covered twice and fills 1 to 3 cover only days 1 to 81. The PDC falls from about 0.833 to 141 / 180, about 0.783, below 80% (computed here for illustration).
Shifting overlaps between different drugs in a PDC class measure
Two different drugs in a class, each dispensed for 30 days on day 1 and taken together, cover days 1 to 30. Shifting the second to start on day 31 credits 60 covered days where the patient had medicine on 30, which overstates the PDC for the class (computed here for illustration).
Sources
PQA rule for shifting overlapping fills of the same drug
Nau DP. Proportion of Days Covered (PDC) as a Preferred Method of Measuring Medication Adherence. Pharmacy Quality Alliance; undated, cited as 2011 by Raebel and colleagues (2013); archived copy of 27 July 2018. Pages 2 and 3: if prescription fills for the same drug overlap, the start date is adjusted to the day after the previous fill has ended, on the premise that the patient finishes the preceding supply first; arrays for concurrent drugs within a class are not adjusted.
CMS same-drug definition and class overlap examples for the PDC
Centers for Medicare & Medicaid Services. Medicare 2026 Part C & D Star Ratings Technical Notes. Baltimore: CMS; updated 25 September 2025. Measure D08 general notes: the PDC is adjusted for overlapping prescriptions for the same drug, defined by the active ingredient at the generic name level. Attachment L, Example 3 (Tables L-4 and L-5): no adjustment where lisinopril overlaps captopril.
Canonical Identity
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