Concept Architecture
Concept
Theoretically, Proportion of Days Covered (PDC) is a quantitative measure of medication adherence that estimates the proportion of days during a defined observation period on which a patient has medication available. It is based on pharmacy dispensing records and is founded on the principle that medication availability is a proxy for adherence to prescribed therapy. Unlike Medication Possession Ratio (MPR), PDC avoids double-counting overlapping prescriptions and caps adherence at 100%, making it the preferred adherence metric in many quality measurement programmes and pharmacoepidemiological studies.
Mathematically, PDC is calculated as the ratio of the number of days covered by medication to the total number of days within the observation period. Coverage is determined by constructing a medication possession timeline in which each calendar day is counted only once, regardless of overlapping prescriptions or early refills. The resulting proportion ranges from 0 to 1 and is commonly expressed as a percentage.
In practice, PDC is estimated from pharmacy dispensing or claims databases by mapping dispensing dates and days' supply onto an observation period and counting the number of covered days. It is widely used to evaluate adherence for chronic therapies, monitor healthcare quality, assess the effectiveness of adherence interventions and estimate the economic consequences of medication non-adherence. In health economics, PDC is frequently incorporated into analyses of healthcare utilisation, treatment effectiveness and cost outcomes.
Purpose
Used to quantify medication adherence from dispensing records by estimating the proportion of time that patients have medication available, supporting evaluation of treatment quality, healthcare utilisation and economic outcomes.
Mathematical Formulae
Primary Formula
PDC = (Number of days covered by medication � Number of days in the observation period) ? 100%
Supporting Formulae
As a proportion:
PDC = Number of covered days � Observation period
Adherence indicator:
Adherent = 1 if PDC � 0.80, otherwise 0
Coverage constraint:
PDC � 100%
Related Mathematical Methods
- Medication Possession Ratio (MPR)
- Adherence Rate
- Persistence Analysis
- Survival Analysis
- Pharmacy Claims Analysis
Example
A patient is observed for 180 days and pharmacy dispensing records indicate medication coverage for 162 unique days after accounting for overlapping refills.
Covered days = 162
Observation period = 180 days
PDC = (162 � 180) ? 100% = 90%
The patient is classified as adherent because the PDC exceeds the commonly applied threshold of 80%.
Excel Implementation
| Function | Example Formula | Health Economics Application |
|---|---|---|
| DATEDIF | =DATEDIF(StartDate,EndDate,"d")+1 | Calculates the observation period. |
| COUNTIFS | =COUNTIFS(CoverageRange,TRUE) | Counts unique covered days. |
| IF | =IF(PDC>=0.8,"Adherent","Non-adherent") | Classifies patients according to adherence thresholds. |
| AVERAGE | =AVERAGE(PDCRange) | Calculates mean adherence across a study population. |
| COUNTIFS | =COUNTIFS(PDCRange,">=0.8") | Counts adherent patients for quality reporting. |
VBA (Optional)
VBA can automate construction of medication coverage timelines, account for overlapping prescriptions and calculate Proportion of Days Covered for large pharmacy claims datasets.
Sources
- Andrade SE, Kahler KH, Frech F, Chan KA. Methods for evaluation of medication adherence and persistence using automated databases. Pharmacoepidemiology and Drug Safety. 2006.
- Steiner JF, Prochazka AV. The assessment of refill compliance using pharmacy records. Journal of Clinical Epidemiology. 1997.
- Nau DP. Proportion of Days Covered (PDC) as a Preferred Method of Measuring Medication Adherence. Pharmacy Quality Alliance.
- ISPOR Medication Compliance and Persistence Special Interest Group. Medication Compliance and Persistence: Terminology and Definitions.
- Drummond MF, Sculpher MJ, Claxton K, Stoddart GL, Torrance GW. Methods for the Economic Evaluation of Health Care Programmes.
- ISPOR Good Practice Reports.
Related Concepts (2)
Library
Publications
1
Opportunities and Responsibilities in Pharmaceutical Care — Hepler & Strand, Vol. 47, No. 3 ed., 1990 (American Journal of Hospital Pharmacy)
The seminal paper that defined "pharmaceutical care" as the responsible provision of drug therapy to achieve definite outcomes that improve quality of life — the founding statement of the pharmaceutical care philosophy that reshaped modern pharmacy practice.
Journal ArticleView source →
Frequently Asked Questions (6)
What is proportion of days covered?
A quantitative adherence measure calculated as the proportion of days a patient had access to medication, accounting for overlapping supply more precisely than MPR.
Source: Cramer et al. 2008
What proportion of days with access to medication does proportion of days covered measure?
Proportion of days covered is a quantitative adherence measure based on prescription refills. It is calculated as the proportion of days a patient had access to their medication. It accounts for overlapping supply more precisely than the medication possession ratio, avoiding the overcounting that early refills can cause. It is a quantitative measure, a numerical proportion rather than a subjective judgement. It is closely related to the medication possession ratio, which it refines by handling overlapping supply better. The share of days a patient had access to the drug is what it measures. Cramer and colleagues (2008) set this out.
Source: Cramer et al. 2008
How is proportion of days covered calculated?
Proportion of days covered is calculated as the proportion of days a patient had access to medication, so it is a quantitative adherence measure computed from that proportion, accounting for overlapping supply more precisely than MPR. This calculation as the proportion of days with access defines it. So proportion of days covered is a quantitative adherence measure calculated as the proportion of days a patient had access to medication, accounting for overlapping supply more precisely than MPR By handling overlapping refills more carefully, the measure avoids overstating adherence when early refills pile up, which is why it is often preferred to the older ratio.
Source: Cramer et al. 2008
What does proportion of days covered account for more precisely?
Proportion of days covered accounts for overlapping supply more precisely than MPR, so as a quantitative adherence measure of the proportion of days a patient had access to medication it handles overlapping supply better than the medication possession ratio. This more precise handling of overlapping supply defines it. So proportion of days covered is a quantitative adherence measure calculated as the proportion of days a patient had access to medication, accounting for overlapping supply more precisely than MPR By handling overlapping refills more carefully, the measure avoids overstating adherence when early refills pile up, which is why it is often preferred to the older ratio.
Source: Cramer et al. 2008
What kind of measure is proportion of days covered?
Proportion of days covered is a quantitative adherence measure, so it is a numerical adherence measure calculated as the proportion of days a patient had access to medication, accounting for overlapping supply more precisely than MPR. This character as a quantitative adherence measure defines it. So proportion of days covered is a quantitative adherence measure calculated as the proportion of days a patient had access to medication, accounting for overlapping supply more precisely than MPR By handling overlapping refills more carefully, the measure avoids overstating adherence when early refills pile up, which is why it is often preferred to the older ratio.
Source: Cramer et al. 2008
How does proportion of days covered relate to the medication possession ratio?
Proportion of days covered relates to the medication possession ratio as a more precise refill-based measure: proportion of days covered is the proportion of days a patient had access to medication, accounting for overlapping supply more precisely than MPR, and the medication possession ratio is the proportion of days within a period a patient had the medication available, based on refill data. So both use refill data, connected in that proportion of days covered handles overlapping supply more precisely By handling overlapping refills more carefully, the measure avoids overstating adherence when early refills pile up, which is why it is often preferred to the older ratio.
Source: Cramer et al. 2008
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