PDC is the medical abbreviation for proportion of days covered, the healthcare industry's standard claims-based measure of medication adherence. It answers a simple question from prescription claims data: out of the days we measured, on how many did the patient actually have medication on hand? A patient with an 80% PDC or higher is generally considered adherent.
If your pharmacy participates in Medicare Star Ratings measures, payer performance networks, or value-based programs, PDC is the number those programs are grading. Here is how it works, how to calculate it, and how it compares to the other adherence measure you will encounter, MPR.
Why PDC matters to your pharmacy
Poor adherence has been associated with worse outcomes, more hospitalizations, and higher total cost of care (Canfield), which is why payers measure it relentlessly. One widely cited estimate puts the health care costs Medicare beneficiaries avoid through the adherence measures at up to $46.6 billion (Schmitz). PDC is the methodology endorsed by the Pharmacy Quality Alliance (PQA), whose adherence measures underpin the CMS Star Ratings for Medicare plans. Three Part D Star Ratings measures, each triple-weighted in the 2026 ratings, track PDC for three chronic medication classes:
- Diabetes medications
- Renin-angiotensin system antagonists (blood pressure)
- Statins (cholesterol)
Plans chase those ratings, and they push the work to the network pharmacies filling the prescriptions. Your pharmacy's aggregate PDC performance can influence network agreements, performance payments, and program eligibility. The National Association of Specialty Pharmacy (NASP) likewise recommends PDC as the preferred adherence measure for specialty pharmacy.
The measure is imperfect by design: claims data can show that a patient had medication available, not that they swallowed it. But because it is objective, standardized, and computable from data every plan already has, it is the common language of adherence across the industry.
How is PDC calculated?
The formula is straightforward:
PDC = (days covered by fills) / (days in the measurement period) x 100
A day is "covered" if the patient had medication on hand that day, based on fill dates and days supply from claims. Two rules make PDC stricter and more accurate than simpler measures:
- Overlaps shift forward. If a patient refills early, the new supply starts counting after the current supply runs out, so overlapping days are not double counted.
- The window caps coverage. Supply extending past the end of the measurement period is trimmed. PDC can never exceed 100%.
A worked example
A patient starts Medication X, and we measure a 90-day window from the first fill:
- Fill 1, day 1, 30-day supply: covers days 1 through 30.
- Refill 1 arrives 3 days late on day 34: covers days 34 through 63. Days 31 to 33 are uncovered.
- Refill 2 arrives 7 days early on day 57: the patient still has supply through day 63, so the new fill starts counting on day 64 and would cover through day 93. The window ends at day 90, so it counts for days 64 through 90.
Days covered: 30 + 30 + 27 = 87 of 90 days, a PDC of 96.7%. The three late days are the only gap; the early refill neither helps nor hurts.
PDC vs. MPR: what's the difference?
Before PDC became the standard, the common measure was MPR, the medication possession ratio:
MPR = (sum of days supply) / (days in the measurement period) x 100
MPR simply adds up all the days supply dispensed and divides by the measurement interval. It does not shift overlapping supply forward, which means early refills inflate the score, and under conventions that measure from first fill to the end of the last fill's supply, MPR can exceed 100%.
Run our example through MPR over the same 90-day window: the patient received 90 days of supply, so MPR is 90 / 90 = 100%. A perfect score for a patient who genuinely missed three days. Run it under the other common convention, which measures from the first fill through the end of the last fill's supply, and the denominator shrinks to 86 days: MPR = 90 / 86 = 104%. An adherence rate above 100% is the overcounting problem in a single number (Schmitz). PDC reads 96.7% either way and catches the gap, because it counts the days supply was actually on hand rather than the total dispensed. MPR conventions vary, so any MPR figure should state the denominator it used; that looseness and inflation are the main criticisms of MPR in the research literature (DeClercq, Nau) and the main reason PQA, CMS, and NASP standardized on PDC.
Both measures share one limitation: they measure possession, not ingestion. Neither can tell you whether the medication was taken, only whether the patient had it.
What is a good PDC score?
The consensus threshold, used by PQA and the Star Ratings measures, is 80%: a patient at or above 80% PDC is classified as adherent for chronic medications. The bar is higher for some therapies; for antiretrovirals, the clinical literature and specialty pharmacy programs commonly use a 90% threshold because the clinical cost of missed days is higher, though that figure comes from HIV research rather than the Star Ratings measures.
Remember that programs score the percentage of your patients above the threshold, not your average PDC. Moving a patient from 76% to 81% changes your measure; moving one from 90% to 95% does not.
How pharmacies improve PDC
Because PDC is computed from fill dates, the levers are operational:
- Medication synchronization, so a patient's chronic fills come due together and gaps stop appearing one drug at a time.
- Refill outreach before the gap, not after: a text or call in the days before supply runs out beats a letter two weeks later.
- 90-day fills where the plan allows, which mechanically reduce the number of refill opportunities to miss.
- Addressing the reason, not just the refill: cost, side effects, and regimen complexity are adherence problems wearing a refill-gap costume.
This is patient operations work, and it is exactly the kind of work DocStation structures. Payer adherence programs surface as task workflows with the patient context attached, non-adherence outreach is a built-in clinical workflow, and two-way text messaging lets your team reach patients where they respond. The care you document supports the claims you bill for adherence consultations, and performance programs like these sit alongside the quality work covered in our guide to quality improvement initiatives in pharmacy.
Frequently asked questions
What does PDC stand for in pharmacy?
The acronym stands for proportion of days covered: the percentage of days in a measurement period on which a patient had their medication available, calculated from prescription claims.
What is a good PDC score?
80% or higher is the standard adherence threshold for chronic medications in PQA and Star Ratings measures; clinical literature commonly uses 90% for antiretrovirals.
How is PDC different from MPR?
MPR divides total days supply by the interval and can exceed 100% when patients refill early. PDC shifts overlapping supply forward and caps at 100%, making it the more accurate and now-standard measure.
Does a high PDC mean the patient is taking their medication?
Not necessarily. PDC measures whether claims data shows medication on hand, not ingestion. It is the best objective proxy available at scale, which is why payers use it.
References
- Canfield SL, et al. "Navigating the Wild West of Medication Adherence Reporting in Specialty Pharmacy." J Manag Care Spec Pharm. 2019;25(10):1073-1077. doi:10.18553/jmcp.2019.25.10.1073
- DeClercq J, Choi L. "Statistical considerations for medication adherence research." Curr Med Res Opin. 2020;36(9):1549-1557. doi:10.1080/03007995.2020.1793312
- Loucks J, et al. "Proportion of days covered as a measure of medication adherence." Am J Health Syst Pharm. 2022;79(6):492-496. doi:10.1093/ajhp/zxab392
- Nau DP. "Proportion of Days Covered (PDC) as a Preferred Method of Measuring Medication Adherence." Pharmacy Quality Alliance.
- Schmitz, Richard. "Medication Adherence Measures Help Medicare Beneficiaries Avoid Up to $46.6 Billion in Health Care Costs." Pharmacy Times.
- Centers for Medicare and Medicaid Services. 2026 Star Ratings technical notes.
- "Do You Know the Difference Between These Adherence Measures?" Pharmacy Times, July 2015.
Turn adherence into a program, not a scramble
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