Examining Thresholds for Diabetes, Renin-Angiotensin System Antagonist, and Statin Medication Adherence Quality Measures: The Application of the Law Of Diminishing Returns in Administrative Claims

Abstract

Objective: 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 1) antidiabetic, 2) renin-angiotensin system antagonists (RASA), and 3) statin medication adherence, at deciles of PDC, with disease-specific and all-cause economic outcomes (inpatient utilization and total healthcare costs) 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 diabetes, RASA, and statin medication adherence measures with non-capitated health plans. Generalized linear models (GLMs) with log link and gamma (costs) or negative binomial (utilization) distributions were used to assess the relationship of adherence with economic outcomes while adjusting for covariables (e.g., age, gender, Charlson comorbidity index). An alpha level of 0.01 was set a priori. Beta coefficients were used to compute use ratios 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 404,108 (diabetes), 1,329,576 (RASA), and 1,266,066 (statin) individuals were included in the study cohorts. Of the 120 GLMs that assessed the relationship between adherence and economic outcomes, 116 significant associations were identified (all p<0.0001). Of these, 98 models identified that adherence was associated with lower cost and utilization compared to nonadherence. Eighteen models showed adherence was associated with higher healthcare costs than nonadherence. The following adherence thresholds were identified as the optimal range of medication adherence thresholds (i.e., points of diminishing marginal and maximum returns): between 86% and 91% PDC for diabetes, 83% and 89% PDC for RASA, and 90% and 96% PDC for statin medications. Conclusions: The law of diminishing returns can be successfully applied to medication-taking behavior to derive optimal adherence thresholds in a commercially-insured patient population. Reliance on the 80% PDC adherence threshold should be re-evaluated to optimize benefits of diabetes, RASA, and statin medication adherence. The application of the law of diminishing returns in other patient populations and medication classes is warranted.