How to Calculate Medication Possession Ratio (MPR)
Medication Possession Ratio (MPR) is one of the most widely used refill-based adherence indicators in pharmacy operations, outcomes research, and payer quality programs. It converts dispensing records into a normalized percentage that answers a practical question: what share of time did the patient have medication available during the measurement window? Because it is simple, transparent, and scalable, MPR is frequently used for baseline adherence monitoring before teams progress to more complex persistence models.
In this guide, you will learn a technically rigorous method to calculate MPR, control data quality, and interpret results without over-claiming clinical meaning. For adjacent analytics, review mean kinetic temperature, clinical trial enrollment duration, and the TTR calculator to compare adherence and control quality views.
Definition and analytical intent
MPR measures medication availability from pharmacy fills, not ingestion. In operational terms, it is the ratio between adjusted days of supply and the number of days in the observation period. Most reporting frameworks express MPR as a percentage and cap values at 100%. The cap avoids misleading interpretations where early refills inflate the ratio above complete coverage.
The metric is useful when teams need a standardized adherence signal from claims or dispensing data at scale. It is less suitable for short interventions where precise dose timing matters, or for therapies with frequent regimen changes that make simple refill arithmetic unstable.
Variables, units, and notation
- Dsupply: total dispensed days of therapy (days).
- Doverlap: overlapping or duplicated supply to subtract (days).
- Dperiod: measurement window length (days).
- MPR: medication possession ratio (percent).
Keep all time values in calendar days and document whether hospitalization days, formulary switches, or payer eligibility gaps are included in the denominator. Without this documentation, comparisons across cohorts become unreliable.
Formula set and stepwise workflow
Adjusted supply: Dadj = Dsupply − Doverlap
Raw MPR: MPRraw = (Dadj / Dperiod) × 100
Reported MPR: MPR = min(MPRraw, 100)
1. Define the measurement period
Start with a clear interval, such as 180 days, 270 days, or 365 days. The period should match the reporting objective. Population quality dashboards usually use annual windows, while intervention pilots may use quarterly windows.
2. Aggregate total days supplied
Sum days of supply from all qualifying fills for the index medication or class. Exclude reversed claims and non-adherent data artifacts. If your system captures partial fills, include exact day equivalents.
3. Correct for overlap and carryover
Early refills can create duplicate coverage days. Subtract overlap days when your governance model requires conservative availability estimates. Apply a single overlap policy across the entire dataset to preserve comparability.
4. Compute and cap MPR
Divide adjusted supply by period days, multiply by 100, then cap at 100%. Round to two decimals for publication so finance, clinical, and analytics teams reconcile the same number.
Validation checks and technical limits
Validate source data by checking fill chronology, impossible negative values, and denominator consistency. Flag records where Dsupply is zero but prescriptions exist, because these often indicate adjudication gaps. Confirm Doverlap never exceeds Dsupply.
MPR is a possession metric, not a pharmacodynamic measure. A high MPR does not guarantee therapeutic control, and a low MPR does not automatically imply treatment failure. For anticoagulation, pair MPR with control-focused metrics such as TTR; for chronic cardiometabolic care, combine MPR with persistence, lab trajectories, and clinical follow-up intervals.
Worked examples
Example A: Dsupply = 270 days, Doverlap = 10 days, Dperiod = 300 days. Dadj = 260. MPR = (260 / 300) × 100 = 86.67%. This exceeds a common 80% screening threshold.
Example B: Dsupply = 150 days, Doverlap = 0 days, Dperiod = 210 days. MPR = 71.43%. This indicates lower refill coverage and may justify outreach, refill synchronization, or benefit design review.
Embed: Medication possession ratio calculator
Use the calculator below to compute MPR with consistent rounding, overlap handling, and optional target benchmarking.