Accountable Care Organization Efficiency on Entry and Shared Savings Bonuses.

Importance: Spending benchmarks in the Medicare Shared Savings Program previously only considered an accountable care organization's (ACO) historical spending, potentially disadvantaging efficient organizations in favor of inefficient ones. To more sustainably reward efficient ACOs, benchmark calculation has evolved, such as the incorporation of average regional spending in 2017, but how benchmarking policy, and its changes, have affected the financial performance of ACOs across the efficiency spectrum remains unclear. Objective: To measure the association between ACO efficiency on entry (ie, the ratio of observed to expected spending) with earning a shared savings bonus. Design, Setting, and Participants: This cross-sectional study used a 20% national sample of Medicare claims to identify ACOs that participated in the Shared Savings Program for a minimum of 4 years between January 2013 and December 2020. ACOs were sorted in quartiles based on their observed to expected spending ratio in their first year of participants. Data analysis was conducted from July 2024 to May 2025. Exposure: Quartiles of ACO efficiency with a higher spending ratio denoting lower efficiency. Main Outcomes and Measures: The primary outcome was receipt of a shared savings bonus in the second, third, and fourth agreement years. Multivariable logistic regression was used to estimate the association between ACO efficiency quartiles and earning a bonus and how the regional benchmark adjustment in 2017 affected this association across measured agreement years. Results: Across 402 ACOs, the median (IQR) spending ratio was 1.000 (0.993- 1.005). After adjustment, the most efficient ACOs (ie, lowest quartile of the spending ratio) had an increased probability of earning a bonus from 24.4% (95% CI, 15.3%-33.4%) to 45.2% (95% CI, 35.4%-55.0%) after the 2017 introduction of the regional benchmark adjustment. However, the least efficient ACOs (ie, top quartile of the spending ratio) were significantly more likely to earn bonuses prior to (43.8%; 95% CI, 33.7%-53.9%) and after (60.7%; 95% CI, 51.3%-70.1%) the benchmarking change, without evidence that this gap narrowed. Conclusions and Relevance: In this cross-sectional study, ACOs across the range of efficiency on entry had greater odds of earning bonuses after the introduction of the regional benchmark adjustment in 2017. However, less efficient ACOs had significantly greater odds of earning bonuses compared with more efficient ACOs, before and after the policy change.