OBJECTIVES

Despite the limited evidence, a threshold of 80% proportion of days covered (PDC) is used to categorize medication adherence. The objective of this study was to assess the association of adherence to renin-angiotensin system antagonist (RASA) medications, defined at deciles of PDC, with disease-specific and all-cause economic outcomes to identify optimal medication adherence thresholds using the law of diminishing returns.

METHODS

This retrospective cohort study included individuals from the Truven Health MarketScan® Commercial Claims and Encounters Research Databases (2010-2012) eligible for inclusion in the Pharmacy Quality Alliance RASA medication adherence measure with non-capitated health plans. Generalized linear models were used to assess the relationship of adherence with economic outcomes (i.e., inpatient utilization and total healthcare costs) while adjusting for covariables (i.e., age, gender, Charlson comorbidity index). Beta coefficients were used to compute utilization and cost ratios and plotted to generate use and cost reduction functions. Marginal use and cost reduction curves were estimated and points of diminishing marginal returns and maximum returns were identified.

RESULTS

A total of 1,329,576 individuals were included and most were middle aged (91%), male (54%), and had preferred provider organization insurance (71%). PDC decile-defined adherence definitions of greater than or equal to 45%, 64%, 75%, 83%, 89%, 92%, 95%, 97%, and 99% PDC were assessed. All definitions were associated with lower utilization and costs compared to nonadherence (all p<0.0001). Adherence of 83% and 89% PDC was found to optimize the economic benefits associated with RASA medication-taking behavior. These thresholds were associated with 45.5% to 40.3% lower disease-specific and 53.5% to 54.8% lower all-cause utilization, and 9.1% to 13.6% lower disease-specific and 17.2% to 17.4% lower all-cause expenditures, compared to nonadherence respectively.

CONCLUSIONS

Adherence between 83% and 89% PDC was found to optimize the economic benefits of RASA medication-taking behavior. Reliance on 80% PDC threshold should be re-evaluated.