Medication adherence plays an important role in the practice of pharmacy and is affected by various factors.

It is defined as “the degree or extent to which the patient conforms to the medication use recommendations specified by the prescriber” (DeClercq).

Poor medication adherence has been associated with negative health outcomes, increased hospitalizations, and higher overall healthcare costs (Canfield). Many different measures of adherence have been developed, and one of them is the proportion of days covered (PDC medical abbreviation). PDC is a commonly utilized, claims-based measure of adherence (DeClercq) Metrics such as the PDC are increasingly being used in the practice of pharmacy, making it important to understand how it is calculated and the impact it can have on pharmacy practice. In this article, we’ll cover the vital information you need to know about PDC measure of adherence, how it can be calculated, and how it compares with other adherence measures.

As a measure of medication adherence, PDC is widely used throughout the healthcare industry in both research and practice settings. There are variations and challenges that exist when it comes to how PDC is calculated; however, it is found to be operationally defined more consistently when compared to other adherence measures such as medication possession ratio (MPR)(Nau).

The Pharmacy Quality Alliance (PQA), is a national organization committed to improving medication safety, adherence, and appropriate use. PQA has endorsed and prefers the PDC methodology for their performance measures (Nau). These measures include the Centers for Medicare and Medicaid Services three Star Ratings that evaluate adherence to medications for diabetes, hypertension, and cholesterol (Schmitz). In addition to endorsement by PQA, the National Association of Speciality Pharmacy (NSAP) also recommends PDC as the preferred method for measuring medication adherence (Loucks).

What Does PDC Stand for in Pharmacy?

PDC adherence measures are necessary to understand as a pharmacist because they are more commonly being utilized to assess pharmacy performance. This can impact access to payer network agreements, direct and indirect remuneration fees, and accreditation scoring (Canfield). Aside from pharmacy performance, adherence is an important benchmark for quality patient care. Pharmacists are uniquely positioned to impact patient adherence to medications and a healthy lifestyle, both of which are significant drivers in prevention and control strategies related to chronic health conditions.

How is PDC Calculated

The PDC calculation utilizes claims data and is expressed as a ratio of “the number of days covered by the prescription fills over the duration of the observation window”(DeClercq). When utilizing PDC, a “covered” day denotes that the prescription medication was available to the patient for administration on that day. Due to the complexity and variability found among parameters used to calculate PDC, the utilization of statistics software facilitates this process (Canfield).

In general; however, PDC can be defined through the use of a simplified equation. The PDC calculation is done by taking the number of days in a period where a patient is “covered” with medication and dividing that by the total number of days in that period (“Do You Know the Difference,” 2015)

PDC = (number of days covered) / (total days in time period) x 100

The PDC is more accurate when compared to other measures of adherence, such as the MPR. The reason is because the PDC takes into account the days covered. This removes the risk of including overlapping days from early refills and shifts the excess supply forward. At the end, the excess is cut from the total supply removing the possibility of a PDC greater than 100% (Loucks).

PDC vs. MPR Adherence: What’s the Difference?

Along with PDC, MPR is another metric that can be utilized to estimate medication adherence. Both PDC and MPR take prescription claims data into account, and while neither can confirm if a medication was actually taken by the patient, they can both objectively be used to provide details of how often a patient had medication available (Canfield).

One of the major differences between PDC and MPR is in how they are calculated. As described above the PDC medication adherence calculation considers the days that are “covered”. Conversely, MPR is calculated by taking the sum of the days’ supply for all fills divided by the number of days in a particular time period (Schmitz).

MPR = (sum of days supply) / (total days in time period) x 100

Unlike the PDC calculation, MPR simply takes the summation of the days’ supply of medication refills across an interval. MPR has received criticism by researchers for its likelihood of overestimating adherence due to early refills being included in the numerator resulting in an MPR that is greater than 100% (Nau)

MPR, similarly to PDC, is lacking a standardized methodology for calculation. While variations in time frame definitions and inconsistency with reporting values over 100% commonly exist, MPR ratios can be calculated using basic spreadsheet software tools (Canfield).


Let’s take a look at a patient example and how adherence would be calculated using MPR and PDC. A patient brings a new prescription (Medication X) to your pharmacy to have it filled. The medication is filled for the first time for a 30 day supply. They refill Medication X for another 30 day supply but are 3 days late for refill 1. They refill Medication X again for a 30 day supply but this time they are 7 days early for refill 2. What would the MPR and PDC be for Medication X?

The MPR calculation would result in an adherence value that is overestimated at 104%. Conversely, the PDC calculation would account for the initial late refill that occurred, and also shift the overlapping days from the early refill forward and cut excess days at the end. Taking the late and overlapping days into account results in a more accurate adherence rate of 96%.

MPR = (30+30+30)/86 (100) = 104%

PDC = (30+27+30)/90 (100) = 96%

PDC Adherence Threshold

The PDC measure is frequently used as a performance measure of adherence by health plans, pharmacies, and researchers. Based on the available literature PQA recommends a PDC adherence threshold of 80% or higher. A patient with this percentage is considered to be highly adherent for classes of chronic medications, and a threshold of 90% is considered adherent for antiretrovirals (Nau).

Final Thoughts

PDC and MPR pharmacy adherence metrics are key measures being utilized by various stakeholders in the practice of pharmacy. Understanding the basics of PDC equips pharmacists to better understand how these adherence measures are being used to demonstrate quality differences among pharmacies, along with the information they continue to provide surrounding medication adherence and the critical role it plays with healthcare outcomes.

To learn more about PDC pharmacy metrics, contact DocStation. DocStation is a cloud-based practice management platform that helps health plans and pharmacies run value-based programs, enabling all stakeholders real-time insights into patient-level, pharmacy-level, and plan-level adherence calculations.


Canfield, Scott L et al. “Navigating the Wild West of Medication Adherence Reporting in Specialty Pharmacy.” Journal of managed care & specialty pharmacy, vol. 25, no. 10, 2019, 1073–1077. doi:10. 18553/jmcp.2019.25.10.1073. Accessed May 10, 2022.

DeClercq, Josh, and Leena Choi. “Statistical considerations for medication adherence research.” Current medical research and opinion, vol. 36, no. 9, 2020, 1549–1557. doi: 10. 1080/03007995.2020.1793312. Accessed May 10, 2022.

“Do You Know the Difference Between These Adherence Measures?” Pharmacy Times, July 6, 2015, Accessed May 10, 2022.

Loucks, Jennifer et al. “Proportion of days covered as a measure of medication adherence.” American Journal of Health-System Pharmacy: AJHP: official journal of the American Society of Health-System Pharmacists, vol. 79, no. 6, 2022, 492–496. doi: 10.093/ajhp/zxab392. Accessed May 10, 2022.

Nau, David P. “Proportion of Days Covered (PDC) as Preferred Method of Measuring Medication Adherence.” Accessed May 10, 2022.

Schmitz, Richard. “Medication Adherence Measures Help Medicare Beneficiaries Avoid Up to $46.6 Billion in Health Care Costs.” Accessed May 10, 2022.