What MPR Medication Possession Ratio Means
The MPR medication possession ratio is a pharmacy-based measure of medication adherence. It estimates the proportion of days a patient has their medication available for use during a given observation period. Health plans, pharmacists, and researchers rely on MPR to identify gaps in treatment and to predict outcomes tied to chronic disease management.
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MPR belongs to a family of adherence metrics that includes PDC (proportion of days covered). While PDC has become the preferred metric in many pharmacy benefit programs, MPR remains widely used because of its straightforward calculation and its applicability to medications with short refill windows.
How MPR Is Calculated
MPR is calculated by dividing the total days' supply of medication obtained during a measurement period by the number of days in that period. The result is expressed as a decimal or percentage.
Basic formula: MPR = Total days' supply ÷ Number of days in the measurement period
In practice, pharmacy claims data are used to sum the days' supply for each refill. If a patient fills a 30-day supply on day one and another 30-day supply on day 28, the overlapping days are typically capped so the numerator does not exceed the length of the observation window.
Interpreting MPR Scores
An MPR of 1.0 (or 100%) means the patient had medication available every day of the measurement period. In real-world pharmacy data, an MPR above 0.80 is often considered good adherence, while scores below 0.50 signal substantial gaps.
Common interpretation bands used in pharmacy research include:
- MPR ≥ 0.80: adherent
- MPR 0.50 to 0.79: partial adherence
- MPR < 0.50: non-adherent
These thresholds vary by clinical context. For drugs with narrow therapeutic windows or for conditions such as epilepsy and HIV, even higher MPR values may be required to prevent serious outcomes.
When MPR Is Used
MPR is most commonly applied to chronic medications where daily or regular use is expected. Typical use cases include:
- Antihypertensives and statins for cardiovascular risk reduction
- Oral diabetes medications and insulin adjuncts
- Antiretroviral therapy for HIV
- Antiepileptic drugs
- Antipsychotics for schizophrenia and bipolar disorder
Payers use MPR to monitor population-level adherence, to design interventions, and to evaluate the impact of adherence-support programs. Researchers use it as an exposure variable in observational studies linking medication taking to hospitalization, emergency department visits, and mortality.
MPR vs PDC: Key Differences
MPR and PDC measure similar constructs but handle early refills and overlaps differently. PDC caps the numerator at the length of the observation period and avoids inflating adherence when a patient refills early. MPR can produce scores above 1.0 if early refills are not capped, which is why many analyses apply a cap of 1.0.
| Feature | MPR | PDC |
|---|---|---|
| Calculation basis | Days' supply ÷ observation days | Days covered ÷ observation days |
| Early refill handling | Can exceed 1.0 if uncapped | Capped at 1.0 by design |
| Sensitivity to short supply gaps | Moderate | Higher |
| Common use setting | Research and some payer programs | Commercial pharmacy performance |
Limitations of MPR
MPR does not confirm that a patient actually took the medication. It assumes that each day of supply corresponds to a day of use, which may not hold for drugs taken intermittently, for PRN therapies, or for medications with long half-lives where missed doses do not immediately create a gap. MPR also depends on pharmacy claims data, so patients who obtain medications from multiple pharmacies or through mail-order may appear more adherent than they are if duplicate fills are not detected.
For short-term therapies and antibiotics, MPR can be misleading because the measurement window is brief and refill timing is less predictive of long-term behavior. In these cases, prescriber assessment and patient self-report remain important complements to pharmacy-based ratios.
Improving MPR Through Pharmacy Practice
Pharmacists can support higher MPR scores by synchronizing refill dates, offering 90-day supplies when appropriate, and using proactive outreach when a refill is late. Medication synchronization programs, blister packaging, and adherence packaging have been shown to increase MPR for patients on multiple chronic medications.
Clear counseling on the purpose of each medication and on what to do after a missed dose helps align patient behavior with the assumptions underlying MPR calculations. When MPR is shared with patients as a feedback metric, it can open a conversation about barriers such as cost, side effects, or complex regimens.