Part 2: Adherence Monitoring: Targeted Categories and Measurements

In our last look at the subject of medication adherence, we examined how plan benefit decision makers are thinking differently about prescription drug use and pharmacy spending, including an increased focus on:

  • Overall medical costs
  • Awareness of the detrimental effects of medication non-adherence
  • Consideration of plan design tactics intended to close gaps in therapy and lower medical costs

With an estimated one third to one half of all U.S. patients not taking medication as prescribed, problems with drug adherence are not isolated to one or even a few therapeutic categories.1 Given finite resources, plans are concentrating adherence improvement efforts on those conditions that are chronic, highly prevalent and costly. 2 In most cases this includes conditions such as asthma, congestive heart failure, depression, diabetes, hypertension, osteoporosis and high cholesterol. Tactics employed include increased education and awareness, simplification of the medication regimen through Medication Therapy Management and/or refill synchronization, case management, and removal of barriers to access (lowering co-pays) to increase adherence to therapy and improve medical outcomes.1, 3

Pharmacy claims analysis is used to measure adherence changes over time after one or more of the above plan design elements are implemented. Historically, the most common measurement of adherence was the Medication Possession Ratio (MPR). In simplest terms, the MPR is the computation of the days supply dispensed to the patient divided by the measurement period, typically one year, with a maximum value of “1.” For example, for a 360-day measurement period, if a patient filled the first two months (60 days) of cholesterol-lowering “statin” medication and then stopped taking it, the MPR would be 60 / 360 = 0.17, obviously non-adherent. A patient that missed only several doses every month but remained on the medication may have a MPR like this: 330 / 360 = 0.92, or adherent. The industry-accepted adherence threshold in most therapeutic categories is 0.8. Patients with a calculated MPR below the threshold are deemed not optimally adherent, while patients with an MPR at or above the threshold are considered to be highly adherent to therapy.4

A newer, alternative adherence measure is the Proportion of Days Covered, or PDC. While the PDC for patients using only one drug is predicted to be identical to the calculated MPR in most cases, PDC is expected to be a more conservative and accurate measure of patient adherence in more complicated cases where a patient is on multiple therapies within a category, or switches amongst multiple drugs over time. The same threshold of 0.8 applies to evaluation of PDC values across patients or populations.4

For those plans enrolled in the Serve You Adherence Monitoring Program, the analytics described above are performed prior to implementation and one time yearly to measure impact of the program over time.

To see an example of this reporting, or to have baseline analytics performed for your plan, please see your Serve You Account Manager.


  1. New England Healthcare Institute. Thinking outside the pillbox. A system-wide approach to improving patient medication adherence for chronic disease. Published 2009. Accessed May 27, 2015.
  2. Roebuck CM, Liberman JN, Medication adherence leads to lower health care use and costs despite increased drug spending. Health Affairs. 2011; 30(1): 91-99
  3. Trompeter E, Maas A. Improving adherence and containing Rx costs: new health plan/PBM strategies. Atlantic Information Services, Inc. Published 2011.
  4. Nau DP. Proportion of days covered (PDC) as a preferred method of measuring medication adherence. Accessed May 27, 2015.