Medication Adherence Measurement for MTM Services
This calculator supports CMS Star Ratings adherence measures and Medication Therapy Management (MTM) programs. PDC (Proportion of Days Covered) is the preferred CMS metric for adherence. The 80% threshold is used for quality measures and pay-for-performance programs.
CMS-preferred method - measures percentage of days patient has medication available
Sum all days supply, excluding overlap
PDC Calculation Method:
PDC = (Total days covered / Days in measurement period) × 100. Overlapping days are counted only once. This prevents inflated adherence from early refills.
Calculate PDC from actual prescription fill dates
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Introduction
Non-adherence to prescribed medications is one of the most costly and preventable problems in healthcare. According to the American Journal of Medicine, non-adherence is estimated to cause approximately 125,000 deaths annually in the US and account for 10% to 25% of all hospital and nursing home admissions, at an estimated cost of $100 billion to $300 billion per year. The Medication Possession Ratio (MPR) and Proportion of Days Covered (PDC) are the two validated metrics pharmacy benefit managers, health systems, and pharmacists use to measure medication adherence from pharmacy fill data. An MPR or PDC below 80% is the widely accepted clinical threshold below which adherence is considered inadequate for chronic disease medications. A patient with hypertension on lisinopril who refills their 30-day supply every 45 days has a PDC of 67%, which clinical evidence links to 35% higher cardiovascular event risk compared to PDC above 80%.
What This Calculator Does
This calculator computes two validated adherence metrics: Medication Possession Ratio (MPR) and Proportion of Days Covered (PDC). Inputs include the number of days in the observation period, number of refills dispensed, days supply per fill, and dates of first and last dispensing. It outputs MPR percentage, PDC percentage, adherence classification (Adherent above 80%, Partially Adherent 60% to 79%, Non-Adherent below 60%), and the number of missed days over the observation period. It also calculates the refill frequency needed to achieve 80% adherence, which is useful for patient counseling.
The Formula
MPR is calculated by summing all days supply dispensed across all fills in the observation period and dividing by the total number of days in the period. MPR can exceed 100% if patients stockpile medication by refilling early. PDC is more precise and is preferred by PQA and health plans for quality measurement: it counts the number of calendar days in the observation period during which the patient had medication on hand, counting each day only once even if multiple fills overlap. For PDC, if a refill is picked up before the prior fill is exhausted, the excess days do not count until the earlier supply runs out. PDC is always between 0% and 100%.
Step-by-Step Example
Define the observation period and gather fill data
Observation period: 365 days (January 1 to December 31). Drug: metformin 500 mg twice daily. Days supply per fill: 90. Fills dispensed: Fill 1 on Jan 1, Fill 2 on Mar 25 (84 days later), Fill 3 on Jul 8 (105 days later), Fill 4 on Oct 10 (94 days later). Total days supply dispensed: 4 × 90 = 360 days.
Calculate MPR
MPR = 360 days supply / 365 observation days = 98.6%. MPR is high, but this does not confirm good adherence because it does not account for the 21-day gap between fill 2 and fill 3 (fill 2 exhausted on June 22, fill 3 not picked up until July 8).
Calculate PDC
PDC maps each day in the period as covered or not. Fill 1: covers Jan 1 to Mar 31 (90 days). Fill 2: dispensed Mar 25, but 7 days of Fill 1 remain, so Fill 2 coverage starts Apr 1 (cover until Jun 29, 90 days from Apr 1). Gap: Jun 30 to Jul 8 = 9 uncovered days. Fill 3: covers Jul 8 to Oct 5. Fill 4: covers Oct 10 to Dec 31 (partial; 5-day gap Sep 6 to Oct 9). PDC = (360 covered days - 14 uncovered days) / 365 = 346/365 = 94.8%. Despite the gaps, PDC is 94.8% due to early refills creating coverage overlap.
Apply clinical threshold and identify intervention need
PDC 94.8% = Adherent (above 80% threshold). If this patient's PDC dropped to 67% due to a single 55-day gap between fills: 365 - 55 = 310 covered days / 365 = 84.9%... No, recalculate: 360 - 55 = 305 days covered / 365 = 83.6%. Still adherent but approaching the threshold. Counsel on consistent refill timing: refill every 85 days for a 90-day supply maintains PDC above 90%.
Real-World Use Cases
CMS Star Rating Adherence Measure Reporting
A pharmacy benefits manager calculates PDC for all diabetes patients on oral medications to report against the CMS Part D Star Ratings adherence measures (STA.01, STA.02, STA.03 for diabetes, hypertension, and cholesterol). The 5-Star threshold is PDC above 80%. For a plan with 2,400 diabetes patients, PDC below 80% in more than 20% of patients means the plan loses 5-Star status and faces $25 to $50 per member per month reimbursement penalties. Each percentage point improvement in plan-level PDC is worth approximately $60,000 in Star Rating-linked reimbursement.
Targeted Refill Counseling for Non-Adherent Patient Identification
A community pharmacy uses MPR and PDC data to identify patients who are non-adherent on antihypertensive medications. Patients with PDC below 60% receive a pharmacist outreach call. The pharmacist reviews barriers: a patient on amlodipine reveals she has been skipping doses due to ankle edema. The pharmacist contacts the prescriber, who switches to an alternative calcium channel blocker. PDC in the following observation period improves from 55% to 89%.
Medication Synchronization Program Impact Analysis
A pharmacy implementing a medication synchronization program (all chronic medications aligned to one monthly pickup) measures the program impact. Pre-synchronization PDC across 180 enrolled patients: average 71%. Post-synchronization PDC: 88%. The 17-point improvement in PDC translates to measurable improvement in Star Rating scores and represents approximately $34,000 in annual reimbursement improvement for a plan with 180 affected patients at $15 per Star Rating point per patient.
Comparison
| PDC / MPR Range | Adherence Classification | Clinical Risk Level | Recommended Action |
|---|---|---|---|
| Above 80% | Adherent | Low | Reinforce positive behavior; maintain monitoring |
| 70-79% | Suboptimal | Moderate | Identify barriers; simplify regimen if possible |
| 60-69% | Partially Non-Adherent | High | Direct pharmacist counseling; address cost/side effect barriers |
| Below 60% | Non-Adherent | Very High | Prescriber notification; medication therapy management review |
Common Mistakes to Avoid
Using MPR as the primary adherence metric when PDC is required. CMS Star Ratings and most health plan quality measures specify PDC, not MPR. MPR can exceed 100% (early fills create apparent stockpiling), which makes it unsuitable for quality measurement. For any CMS or health plan reporting, use PDC. MPR is acceptable for internal pharmacy adherence screening but should not be submitted for quality reporting.
Calculating PDC without capping overlapping fill periods. If a patient picks up a 90-day supply 7 days before the prior fill ends, the overlap period cannot be counted twice in PDC calculation. The covered days for the new fill start the day after the prior supply is exhausted, not on the dispensing date. Software that counts overlapping days twice overstates PDC.
Using a 30-day observation window instead of a minimum 180 to 365-day window. Adherence metrics are only statistically meaningful over longer observation periods. A patient who missed one refill in a 30-day window shows 0% adherence for that month, which is misleading. CMS specifies 365-day observation periods for Star Rating measures. Use minimum 180-day windows for meaningful adherence assessment.
Attributing non-adherence to patient behavior without assessing structural barriers. A patient with PDC of 55% may be skipping refills due to out-of-pocket cost, not indifference. PDC identifies the problem; it does not explain it. Always investigate why before labeling a patient as non-adherent and escalating to prescriber notification.
Frequently Asked Questions
Accuracy and Disclaimer
Adherence calculations in this tool are based on standard Medication Possession Ratio and Proportion of Days Covered methodologies as defined by the Pharmacy Quality Alliance (PQA) and used in CMS Part D Star Rating quality measures. Results reflect fill data accuracy and may not capture medication consumption behavior between fills. PDC does not measure whether the patient is taking the medication correctly, only whether it was dispensed and available. Clinical decisions about medication non-adherence require professional assessment of patient barriers, clinical outcomes, and therapeutic goals. This calculator is for pharmacy professional use and does not constitute individualized medical advice.
Conclusion
Medication adherence measurement is most useful when it leads to a targeted intervention. For patients identified as non-adherent on the MPR or PDC calculation, the next step is understanding why: cost barriers, side effects, complexity of regimen, or lack of perceived benefit. For pharmacy programs tracking adherence at scale, the metrics generated here align with the Pharmacy Quality Alliance (PQA) adherence measures used in CMS Star Ratings, where PDC above 80% in diabetes, hypertension, and cholesterol medications directly affects plan reimbursement. Use the Pharmacy Compounding Cost Calculator if adherence counseling reveals that a patient is struggling with commercial formulation tolerability, which compounding services may address.
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