1981-P: Impact of Medicare Part D Cost Sharing on Utilization of Insulin and Novel Diabetes Medications

Introduction and Objective: The original Medicare Part D benefit included an initial coverage phase (25% cost-sharing or fixed copayments), a coverage gap (“doughnut hole” with 100% cost-sharing), and a catastrophic phase (5% cost-sharing). Transitions between phases serve as natural experiments on how large increases or decreases in cost-sharing influence adherence to diabetes drugs. This study's objective was to examine how benefit phase transitions affect medication adherence among Medicare beneficiaries with type 2 diabetes. Methods: This quasi-experimental difference-in-differences study used 2007-13 5% Medicare claims. The control group included beneficiaries with low-income subsidies (LIS) who faced consistently low cost-sharing (<$6.30). The treatment group included three sets of non-LIS beneficiaries: 1) those who transitioned from catastrophic phase to initial coverage (increased cost-sharing); 2) those who transitioned from coverage gap to initial coverage (decreased cost-sharing); and 3) those who remained in initial coverage phase (similar cost-sharing). Outcomes included adherence (proportion of days covered [PDC]≥0.80) and 30-day treatment gaps for diabetes drugs (GLP-1s, SGLT2-inhibitors, DPP-4s, and insulin). Multivariable logistic regression with generalized estimating equations controlled for sociodemographic factors and comorbidities. Results: Among 45,109 beneficiaries, increased cost-sharing reduced adherence (odds ratio [OR] 0.78, 95% CI 0.70-0.86) and increased 30-day gaps (OR 1.75, 95% CI 1.55-1.98). Conversely, decreased cost-sharing improved adherence (OR 1.38, 95% CI 1.29-1.47) and reduced 30-day gaps (OR 0.77, 95% CI 0.72-0.82). Beneficiaries who had no major cost-sharing changes had similar drug use. Conclusion: Medicare beneficiaries’ adherence to diabetes drugs was highly sensitive to cost-sharing changes. Future research is needed to evaluate the effect of recent reforms to Medicare Part D, including a $35 insulin copayment cap and an annual out-of-pocket cap ($2,000 per year starting 2025). Disclosure P. Li: Consultant; Current; Cobbs Creek Healthcare, SKB Consulting. J. Doshi: Consultant; Current; AbbVie Inc. Consultant; Ended; Acadia Pharmaceuticals Inc., Janssen Pharmaceuticals, Inc. Consultant; Current; Merck & Co., Inc. Research Support; Current; Janssen Pharmaceuticals, Inc., Merck & Co., Inc. Research Support; Ended; Spark Biomedical, Inc. M. Klebanoff: None.