Clinician System Affiliation, Practice Size, and Medicare ACO Participation

The Centers for Medicare and Medicaid Services (CMS) and other policymakers have expressed interest in expanding rates of provider participation in accountable care organizations (ACOs), particularly among smaller physician group practices and independent providers. We described the extent to which clinician-level participation in ACOs varies as a function of both practice size and practice affiliations with health systems. We found that clinicians operating in larger practices and practices affiliated with health systems were more likely to participate in ACOs. Furthermore, engaging the approximately 800 largest practices not enrolled in ACOs as of 2022 in ACOs would increase clinician-level ACO participation rates by nearly 25 percentage points, while achieving the same gains in participation by engaging smaller practices would require engaging over 35,000 smaller practices. Our findings imply that prioritizing independent smaller practices for ACO engagement will yield smaller increases in clinician-level ACO participation rates compared to engaging larger system-affiliated practices.