7. ABSTRACT Most patients requiring dialysis for end-stage kidney disease (ESKD) obtain Medicare, with the majority enrolling in fee-for-service (FFS or traditional) Medicare. Medicare Advantage (MA), when private insurers coordinate Medicare benefits, is an alternative to FFS Medicare. However, prior to 2021, patients with ESKD could only enroll prior to developing ESKD. Consequently, only 22% of Medicare patients with ESKD have an MA plan, compared to 36% of Medicare patients without ESKD. The 21st Century Cures Act opened MA to all patients with ESKD in 2021, and experts anticipate a large influx of enrollment into MA. Proponents of MA argue that it results in better care-coordination of chronic diseases, leading to improved outcomes and reduced costs in the non-ESKD population. It is unclear whether these findings extend to ESKD, and research on MA ESKD outcomes are conspicuously sparse, despite its imminent importance. A key explanation is a lack of available research data. The United States Renal Data System (USRDS), the dataset used for most dialysis research, almost exclusively contains data on FFS Medicare. Additionally, because MA plans and dialysis providers negotiate proprietary contracts for dialysis payment, MA dialysis reimbursements have substantial variation, making comparisons across MA plans and providers difficult. Thus, researchers have focused primarily on FFS Medicare. We will use a novel linkage between MA claims data and the USRDS dataset to investigate outcomes in the MA dialysis population. In addition to being one of the first research projects to broadly study dialysis in MA, our work will develop a research infrastructure that will make future research on MA and ESKD more accessible. A key innovation is identifying a comparable unit of dialysis treatment (and all related services) across MA plans and providers, and with FFS Medicare. In Aim 1, we will investigate whether patients with MA accrue the benefits of improved care-coordination seen in non-dialysis populations with MA. We will study whether MA results in improved outcomes at the start of dialysis (e.g., starting dialysis as outpatients) and whether prevalent patients with MA and ESKD experience fewer hospitalizations or have lower mortality. As a potential mechanism for improved care-coordination, we will explore whether MA patients are more likely to receive extra outpatient dialysis sessions over the conventional thrice weekly dialysis schedule. Aim 2 will assess heterogeneity in the MA market. We will study how market concentration at the dialysis facility, the dialysis chain, and the MA plan levels modify our findings in Aim 1. For instance, dialysis providers with few regional competitors may have more market power and, consequently, might be resistant to care-coordination and cost-saving initiatives by MA plans. The proposed work will form the impetus for a larger research effort, including an R01 application, aimed at studying the impact of the 21st C...