The Difference Between Primary Measures of Medication Adherence: PDC and MPR

Written by Nihar Patel DPT'22, MBA'22
Published on November 15, 2018

Two measures of medication adherence are currently utilized in the widely available research assessing the adherence to prescriptions. The primary measures of medication adherence, PDC (proportion of days covered) and MPR (medication possession ratio), can easily be calculated with the extraction of retail pharmacy data. Both MPR and PDC are sufficient at measuring medication adherence, however, there are differences that exist between them.        

MPR is calculated as the ratio of the amount of days a patient has their medicine on hand to the amount of days a patient is eligible to have the medicine on hand. The ratio is ideal at 100% or 1:1 since the patient has refilled their medications on time and/or they have received all the medications prescribed by the doctor. The MPR is easily accessible since it can be synthesized by obtaining readily available pharmacy data. Although MPR does not accurately measure medication adherence to the extent that the patient is actually taking the medication as directed, it does assess whether the patient has access to the drug which. This is a large part of the medication adherence process. In addition, MPR may be a skewed number if the patient is obtaining medications earlier than needed which will make the ratio greater than 100%. In this case, many studies cap the ratio at 100% so it doesn’t skew other calculations such as average MPR rates for a therapeutic category.

PDC fixes the issue of the possibility of having a skewed MPR by capping the ratio at 100%. The PDC is calculated by the ratio of the number of days the patient is covered by the medication to the number of days the patient is eligible to have the medication on hand. The PDC ratio provides a more accurate representation of medication adherence because it eliminates the possibility of being unreasonably elevated. In certain situations, the MPR can be elevated such as when a patient obtains their medications 10 days in advance. The days they have the medication increases while the days they are eligible to have the medication remains unchanged. The PDC would not be affected in this situation because it counts the number of days covered by the medication in a certain period of time rather than the sum of the days that the medications lasts during this time period.  See Figure for a graphical display of this phenomena.

Calculation of Medication Adherence

Figure.  Calculation of Medication Adherence with MPR and PDC

PDC is recommended for assessing the medication adherence of patients on multiple therapies at the same time. PDC calculates the ratio of number of days the patient is covered by the medication in a period to the total number of days in the period. Therefore, PDC can also calculate the number of days the patient is covered by multiple medications in a period by the total number of days in the period. MPR is unable to cover multiple therapies since its numerator is the sum of days supplied in the period. To calculate MPR for multiple therapies, the MPR of each drug must be averaged. This may lead to skewed results because in a situation with MPRs over 100%, the baseline average will increase which will result in a higher average MPR than in reality. Although both PDC and MPR are used to assess medication adherence, PDC is the preferred method for assessing adherence by the Pharmacy Quality Alliance (PQA) for use in Medicare Star Ratings.

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