How medication adherence is quantified when multiple medications are involved?

The evaluation of 147 studies revealed that 66 measures were used to calculate multiple medication adherence, of which, self-reported multi-/single-item questionnaires or scales, Proportion of Days Covered (PDC), Medication Possession Ratio (MPR) are most commonly used measures. Interestingly, formulas of these measures vary widely across studies. Simulation study further showed that ‘MPR/PDC including availability of all medications’ was the most accurate method to measure multiple medication adherence. For more details, read paper at https://pubmed.ncbi.nlm.nih.gov/30711058/

There is no gold standard method to measure medication adherence, but there are several methods available-direct methods which includes measurement of drug or metabolite concentration in the body fluid or direct observation of patient taking medication, indirect methods such as pill counts, measures such as Proportion of Days Covered (PDC) and Medication Possession Ratio (MPR) which are usually reported as percentages of the time when a patient has medication available. The existing methods/measures used to calculate medication adherence were developed considering the use of a single medication, but in real world, patients usually use multiple medications to treat one or more conditions. In that case, currently available measures/methods calculating medication adherence may over- or under-estimate medication adherence. How medication adherence is typically quantified when multiple medications (polypharmacy) are involved? In a report “Methods for Measuring Multiple Medication Adherence: A Systematic Review—Report of the ISPOR Medication Adherence and Persistence Special Interest Group” published in Value in Health, we tried to answer this question. 

The study evaluated 147 studies that reported measures for multiple medication adherence in 13 diseases and found that self-reported multi-/single-item questionnaires or scales are most commonly used to calculate multiple medication adherence (53.1% studies used self-reported method), followed by methods using prescription refill data, electronic medical records, administrative/pharmacy claims data or patient’s appointment records (52.4% studies). About 50% of the studies included more than one method for measuring multiple medication adherence.

The self-reported methods identified for multiple medication adherence used 32 different questionnaires/scales; Morisky Medication Adherence Scale (MMAS 4-/8-/9-item version) being the most applied scale. The measures based on electronic data records included variations of PDC, MPR and missing days or doses calculations. Interestingly, different formulas for PDC, MPR and missing days/doses calculations were used across studies, 10 different formulas of PDC and 9 different formulas of MPR were used while quantifying multiple medication adherence. The selection of an appropriate method or a combination of methods was found to depend on the study setting, data source(s), disease type, and the availability of other resources.

The study also found that “application of adherence measures varied across disorders. For cardiovascular disorders, PDC was the most commonly used adherence measure; nevertheless, for sexually transmitted disorders (e.g., HIV/AIDS), self-reported methods were preferred”.

Simulation study “Measuring Multiple Medication Adherence–Which Measure When?” published in Value Outcomes and Spotlight further illustrated that MPR or PDC including all prescribed medications was the most accurate method to calculate multiple medication adherence. 

There is no standardized measure to calculate medication adherence when multiple medications are involved and there is also no agreement among researchers about how to use existing measures to calculate multiple medication adherence.