Understanding medication adherence metrics: PDC and MPR
Healthcare professionals rely on medication adherence metrics to identify non-adherence, evaluate interventions, and improve patient outcomes, yet different measurement methods can yield conflicting estimates and clinical interpretations.1,2,3
Accurate measurement is foundational: without reliable adherence data, clinicians may misattribute treatment failure to pharmacological ineffectiveness rather than behavioral barriers.1,4 This article clarifies the two most widely used pharmacy claims-based metrics – Proportion of Days Covered (PDC) and Medication Possession Ratio (MPR) – and examines their clinical utility, limitations, and appropriate thresholds in global practice. Understanding these distinctions enables healthcare professionals to select the most valid measure for their setting, interpret pharmacy data correctly, and avoid overestimating adherence when supporting patients with chronic conditions.
What does PDC mean and how is it different from MPR?
PDC and MPR are both pharmacy claims-based metrics that estimate medication availability, but they differ fundamentally in how they handle overlapping refills, leading to systematically different adherence estimates for the same patient.2,3
Proportion of Days Covered (PDC) calculates the percentage of days in a specified period during which a patient has medication available. When a refill is dispensed early, any overlapping supply is shifted forward to begin the day after the previous supply runs out; excess supply beyond the end of the measurement window is truncated. This prevents double-counting and keeps PDC at or below 100%.2,3
PDC = (Sum of days covered in time frame) ÷ (number of days in time frame) × 100 2,3
For example, if a patient refills a 30-day supply on day 25, the overlapping 5 days are shifted forward; only days 1–30 are counted in the numerator, giving a PDC of 100% for a 30-day window.
Medication Possession Ratio (MPR) sums all days’ supply dispensed and divides by the number of days in the measurement period. Because overlapping refill days are included in the numerator without adjustment, MPR can exceed 100% when a patient refills early or accumulates stockpiles.2,3 It is important to note that MPR is not a single calculation: the literature has identified distinct MPR variants, differing in how the denominator is defined (fixed interval vs. first-to-last fill) and how the last fill’s days’ supply is treated. This heterogeneity means that MPR values are not always comparable across studies.2,5
MPR = (Sum of days supply in time frame) ÷ (number of days in time frame) × 100
PDC is endorsed by the Pharmacy Quality Alliance (PQA) and the National Association of Specialty Pharmacy (NASP) as the preferred measure because it provides a more conservative, clinically realistic estimate.3,6,7
MPR tends to overestimate adherence, particularly in patients who stockpile medication through early refills or switch formulations mid-treatment.3,5
When patients use multiple drugs, the choice of adherence definition matters considerably. If adherence is defined as having at least one medication available on any given day, almost all patients appear adherent –
clearly overestimating true regimen adherence. Stricter criteria, such as requiring ≥80% PDC for each drug individually, identify far fewer adherent patients and better reflect full-regimen consistency.8
From a clinical perspective, PDC aligns better with the central adherence question: on how many days did the patient have access to their prescribed therapy? For example, possession on 94% of days (PDC 0. 94) has different clinical implications than possession of 102% of needed doses spread unevenly (MPR 1.02).3 Healthcare professionals reviewing pharmacy reports should confirm which metric is used, recognize that MPR may mask gaps in therapy, and prefer PDC for decision-making when both are available.
What PDC threshold defines “Adherent” (e.g., 80%) and when is it appropriate?
According to the Pharmacy Quality Alliance (PQA), PDC is the preferred method for assessing medication adherence for chronic therapies.3 PQA states that the PDC threshold represents the level at which a medication is reasonably likely to achieve its intended clinical benefit, and for many chronic drug classes, clinical evidence supports a standard threshold of 80%.6,7
However, PQA recognizes that this threshold is not universal. Certain therapies require higher adherence levels, and PQA’s own PDC: Antiretroviral Medications (PDC-ARV) measure uses a 90% threshold, reflecting the need for more consistent medication coverage in this therapeutic class where even brief lapses can compromise viral suppression.6,7
PQA also clarifies that PDC is most appropriate for long-term chronic treatments and may not be suitable for therapies of shorter duration. In such cases, PQA employs alternative measurement approaches, such as evaluating therapy completion (e.g., the Hepatitis C Completion of Therapy measure) or assessing treatment persistence (e.g., the Persistence to Basal Insulin measure).6
In summary, within the PQA framework, PDC ≥0.80 is a clinically supported standard for many chronic medications, but higher thresholds or different adherence methodologies are used when the therapeutic context requires them. PQA positions the threshold not as a universal rule but as a therapy-specific indicator of likely clinical benefit, selected to ensure accurate and meaningful quality measurement.
Why can “Medication Possession” overestimate true adherence?
Despite their widespread use, both MPR and PDC exhibit important methodological limitations that constrain their accuracy, especially in the context of multiple-medication regimens. As documented by the International Society for Pharmacoeconomics and Outcomes Research (ISPOR) Medication Adherence and Persistence Special Interest Group systematic review, MPR commonly overestimates adherence because it accumulates overlapping days of supply, inflating adherence values even in single-drug regimens.9
Although PDC partially addresses this problem by shifting overlapping supply forward rather than simply accumulating it, it still fails to account for stockpiling at the end of the measurement window and does not capture situations where patients initiate therapy late or discontinue early within the measured period.5,9
When applied to polypharmacy, both metrics introduce further bias: depending on the calculation method chosen, they can mask poor adherence to individual treatments, incorrectly classify patients as adherent when only one medication is on hand, or underestimate clinically meaningful gaps that occur when all required medications are not simultaneously available.8,9
Empirical research using real-world pharmacy databases confirms that different computational variants of PDC and MPR can produce significantly divergent adherence classifications for the same patient, with some variants systematically over- or under-estimating adherence relative to the PQA-endorsed PDC reference measure.5 These disagreements are large enough to shift patients between adherence categories, highlighting the need for transparent and consistent calculation methodology.
Conclusion
Medication adherence metrics such as PDC and MPR play a pivotal role in understanding whether patients have access to – and consistently use – their prescribed therapies. As this article illustrates, these measures are not interchangeable: PDC offers a conservative and clinically aligned estimate of coverage, while MPR is more vulnerable to inflation from early refills and stockpiling, and exists in multiple variants that are not always comparable across studies.
In practice, this means favoring PDC for chronic disease management, adjusting thresholds when therapeutic evidence demands it (e.g., 90% for antiretrovirals), and being especially cautious when assessing adherence in patients taking multiple medications. By understanding the nuances behind these metrics, healthcare professionals can better identify genuine adherence gaps, tailor interventions more effectively, and ultimately strengthen clinical decision-making to support improved patient outcomes.
This article was written with the assistance of generative AI technology and reviewed for accuracy.
FAQ
PDC is a pharmacy claims-based metric that calculates the percentage of days in a defined period during which the patient had medication available. It shifts overlapping supply from early refills forward and truncates excess supply at the end of the window, providing a conservative estimate of adherence capped at 100%. 2,3
PDC shifts overlapping refill days forward so each day is counted only once, while MPR sums all dispensed days’ supply – including overlaps—allowing adherence values to exceed 100%. Because of this, MPR frequently overestimates adherence, especially when early refills or stockpiling occur. 2,3
Organizations such as the Pharmacy Quality Alliance (PQA) and the National Association of Specialty Pharmacy (NASP) endorse PDC because it better reflects actual medication availability and avoids inflation from overlapping refills. It aligns more closely with the clinical question: “On how many days did the patient truly have medication available?” 2,3,6
A PDC ≥80% is widely used as the minimum level of adherence needed for many chronic medications to provide therapeutic benefit. This threshold is evidence-based but not universal – it is a therapy-specific indicator rather than an absolute rule. 6,7
Certain therapeutic classes, such as antiretroviral therapy (ARV), require more consistent coverage. PQA uses a 90% PDC threshold for ARVs because missed doses can rapidly compromise treatment effectiveness and risk viral rebound or resistance development. 6,7
MPR adds together all dispensed days’ supply over a time period and divides by the number of days in that period. If a patient refills early, overlapping supplies are counted cumulatively, producing values over 100%, which can misrepresent true medication-taking behaviour. Multiple MPR variants exist that handle the denominator and last-fill supply differently, making cross-study comparisons unreliable. 2,3,5
Both PDC and MPR assume that possessing medication equals taking medication. They do not account for behaviours such as stockpiling, delayed initiation, or taking medication inconsistently – even when the patient physically has the pills. 2,5,9
In polypharmacy, calculation rules differ on whether adherence requires having any medication available or all required medications simultaneously. Using lenient definitions can falsely classify patients as adherent even if they are missing key medications. Requiring ≥80% PDC for each drug individually is a stricter and more clinically meaningful criterion. 8
