Do You Know the Difference Between These Adherence Measures?


MPR and PDC are the most common measures of medication adherence based on patient refill records.

There are a number of direct and indirect ways to measure medication adherence. Some of the most common indirect measures use the patient refill record.

Using the refill record may be the most appropriate method for a pharmacy that needs to report on patient adherence to a particular drug, or a health plan that wants to measure the adherence of its patient population. Fortunately, both entities have access to prescription claims.

While basing adherence on a patient’s refill history doesn’t account for taking the medication the correct way, it is an objective and relatively easy process compared with other methods such as pill counts and questionnaires. The key variables needed from each claim are the drug name, date filled, and days’ supply. Using this information can determine a patient or population’s medication possession ratio (MPR) and proportion of days covered (PDC).

MPR and PDC are the most common measures of medication adherence using refill records. They are usually reported as percentages of the time when a patient has medication available.

MPR is the sum of the days' supply for all fills of a given drug in a particular time period, divided by the number of days in the time period (Figure 1).

Figure 1. MPR Calculation

This is a relatively simple calculation, but it has its faults. For instance, MPR can overestimate adherence, and in many cases, it does.

Patients who routinely refill their medications early will have an inflated MPR, as the numerator in this equation will be larger than the denominator. Also, the exact formulas used to calculate MPR vary from source to source.

For example, when reporting a population’s MPR, some entities will “cap” individual adherence at 100%. Among entities that do not cap MPRs, the population average will be falsely elevated.

PDC is a newer, more conservative measure of refill record-based adherence. The formula is similar to MPR, but instead of simply adding the days' supplied in a given period, the PDC considers the days that are “covered” (Figure 2).

Figure 2. PDC Calculation

This is a subtle, yet important difference from MPR.

In the MPR calculation, a patient who refills a medication 7 days prior to running out of it will have overlapping days' supplied, which would elevate MPR. But PDC makes an adjustment.

Think of each prescription as an “array” of days' supplied. Before the numbers are crunched for PDC, overlapping arrays are moved forward to the first day that the patient would not have medication from the previous dispensing (previous array). Moving these arrays forward provides a true picture of the days on which a patient is “covered” with medication, rather than a simple summation of all days supplied with MPR. By using days covered, it is impossible to calculate a PDC greater than 100%.

PDC is also better suited for medication regimens, such as antiretroviral therapy for HIV or multiple medications for diabetes. For such regimens, MPR would be calculated by finding the average of the MPRs for each drug for a given patient.

The flaw of this approach is that high-MPR medications can offset poor-MPR medications and lead to an acceptable average for the entire regimen. But the intent of a prescribed regimen is for the patient to be adherent to all medications.

PDC doesn’t simply average the PDC for individual drugs; instead, it considers the days within a particular period when a patient is covered for all medications in a regimen. In other words, for a 3-drug regimen, a day is only considered "covered" when all 3 medications are available to the patient.

Although MPR is more commonly used, PDC is becoming the preferred adherence measurement because of its advantages. The Pharmacy Quality Alliance has endorsed PDC as its recommended measure of adherence, and the US Centers for Medicare and Medicaid Services has incorporated it into its plan ratings. Accrediting bodies such as URAC are even beginning to require PDC in the annual reports of accredited organizations.

Still, there is no real consensus on the optimal level of adherence. In some cases, researchers have concluded that 80% is acceptable in many disease states. However, there are others in which even greater levels of adherence are required to avoid negative outcomes.

For instance, some studies involving HIV medications have found that an adherence level of 95% is required to avoid the risk of drug resistance. To assess adherence among a patient population, simply calculating the average adherence level of the population is simple to do, but a good-looking average can exist even when a significant number of patients have very poor adherence. To correct for this, another method is to predefine an acceptable level of adherence for the population and then measure the percentage of patients who meet or exceed this level.

Some words of caution for those using refill history to measure adherence:

  • All of the patient’s fill records for the assessed drug or regimen are necessary. Otherwise, it may appear as though a patient is not consistently filling their prescriptions, and consequently, the MPR or PDC will be reduced.
  • Entering the incorrect days’ supplied and other human errors can result in inaccurate results.
  • The results can be overstated due to early dose titrations and patients refilling early. Conversely, for patients who are new to therapy, the fill date for the first dispensing can be much earlier than the actual first dose. This might occur because the patient is awaiting administration training from the doctor or tapering off of another medication, which can lead to a later-than-expected first refill and decrease the calculated MPR or PDC.

Despite the challenges and disadvantages of MPR and PDC, these measures continue to be commonly used to gauge patient and population adherence. Because of the advantages of PDC, the use of MPR will dwindle over the next several years. At the same time, the use of technology to more directly measure adherence will increase, giving pharmacists and the health care industry a look beyond refill behavior and into dose-by-dose adherence.

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