Measuring Concurrent Adherence to Multiple Related Medications

Adherence (also called compliance) to prescribed medications is a key dimension of healthcare quality. Nonadherence is associated with poor health outcomes1 and with substantial economic cost2,3 and threatens the gains in quality that have been made by appropriate pharmacotherapy over the past several decades.4 Efforts to accurately measure and improve adherence have received increasing attention from patients, physicians, payers, and other healthcare stakeholders. Moreover, the National Committee for Quality Assurance has recently included adherence among the measures by which it evaluates the quality of care provided by healthcare plans.5 Adherence may be assessed in various ways, including surveying patients, directly observing medication taking, measuring drug or metabolite blood levels, and using electronic medication monitors.3 Pharmacy refill claims, which are highly correlated with these measures,6 provide an objective way to measure adherence for large populations of patients. Such data are frequently used by health services researchers and pharmacy benefit plan managers to calculate adherence by estimating the amount of medication available to a patient over a given interval.7 These measures were developed and have been most widely used to evaluate adherence to individual medications or medication classes. For example, Benner et al8 assessed statin adherence by measuring the proportion of days covered (PDC) for patients newly prescribed 1 of these agents. However, it is common for patients with chronic conditions (such as hypertension, coronary artery disease, or congestive heart failure) to switch between classes of medications or to receive multiple medications to treat a single disease. Such behavior is clinically sensible and formalized in practice guidelines but is inadequately captured by measuring adherence for a single agent or medication class. Studies that have measured concurrent adherence to multiple medication classes have used various techniques,9-11 and unlike adherence to single medications or single medication classes,6,12 there are no published definitions or guidelines about how to structure these measurements. Understanding the variability in adherence estimates obtained from different measures will assist in selecting appropriate definitions. In this article, we propose standardized methods for measuring concurrent adherence, apply them to a cohort of patients with diabetes mellitus receiving treatment with oral hypoglycemic agents, and compare their performances. Take-Away Points Variability in adherence estimates derived from different definitions of concurrent adherence to related medications may lead to very different qualitative conclusions about adherence in a given population. Therefore, it is important to establish how concurrent adherence to multiple related medications should be calculated. This study found that concurrent medication adherence varied widely depending on the definitions applied because of the assumptions on which they are based. The choice of adherence measurement technique should depend on the reason why adherence is being measured.