Research and scholarly methods: Measuring medication adherence

Medication adherence is the extent to which a patient follows prescribed directions when taking a medication. Research examining interventions relating to medication adherence is becoming increasingly important for payers, providers, and researchers to inform clinical practice and utilization policies for medications. Multiple methods exist for measuring medication adherence, and the choice of methodology should be based on research design, data availability, and study aims. In this review, common methods for measuring medication adherence in retrospective studies using claims-based data, prospective studies using data collected by the research team in real-time, and patient/caregiver/clinician reported approaches using survey/interview data are discussed to facilitate research and review of studies involving medication adherence. For retrospective studies, this review discusses metrics designed to calculate medication adherence based on fills or dispense dates and days supplied. Such metrics include medication possession ratio (MPR) and proportion of days covered (PDC). A brief introduction of measuring adherence to multiple medications and group-based trajectory modeling (GBTM) is included. For assessing adherence in prospective studies, a discussion on objective observational methods, such as pill counts and therapeutic drug monitoring and specialized pill bottle or cap devices, is covered. Lastly, for studies assessing patient/caregiver/clinician reported outcomes, a discussion of how diaries, interviews, and surveys/questionnaires/interviews can be used to measure medication adherence is reviewed.