Importance: Accountable care organizations (ACOs) may increase health care disparities by excluding physician groups that care for socially and clinically vulnerable patients. Objective: To estimate the association between the patient characteristics of a physician group and the group's participation in a newly formed ACO. Design, Setting, and Participants: This retrospective cohort study investigated a 20% random sample of US Medicare fee-for-service beneficiaries attributed to physician groups identified in Medicare claims before ACO participation from January 1, 2010, through December 31, 2011. Physician groups that participated and did not participate in the Medicare Shared Savings Program (MSSP) from January 1, 2012, through December 31, 2014, were identified in the Medicare MSSP 2014 provider file. Data analyses were conducted from September 1, 2017, to March 30, 2018. Exposures: Using multivariable regression, the association between physician group participation in the MSSP and the group's patients' characteristics before ACO formation was estimated focusing on measures of the vulnerability of the group's patients. All ACO-participating physician groups were compared with ACO-nonparticipating physician groups for reference, and estimates were made at the physician and patient level. Main Outcomes and Measures: Percentage of a physician group's patient panel that was socially vulnerable (based on race, dual Medicare and Medicaid enrollment, or living in high-poverty zip code) or clinically high risk. Results: Among 67 891 physician groups caring for 5 394 181 patients, 7215 physician groups (10.6%) participated in an MSSP ACO by 2014. Comparing mean percentages across practices, the patients of non-ACO-participating physician groups, more patients of ACO-participating physician groups were black (mean percentage across practices, 12.1% vs 10.6%), dually enrolled in Medicare and Medicaid (23.0% vs 19.3%), living in poverty (10.7% vs 11.1%), and high risk (34.2% vs 30.2%). After adjustment, physician groups that participated in an ACO had 5.1 percentage points (95% CI, 0.1-10.0 percentage points; P = .05) more dually enrolled patients and 4.0 percentage points (95% CI, 1.9-6.1 percentage points; P < .001) more high-risk patients. At the patient level, patients who were at high risk were more likely to be attributed to a group that became part of an ACO, with 4.5 percentage points (95% CI, 0.5-8.5 percentage points; P = .03) more high-risk patients being attributed to an ACO, but other associations were not statistically different from zero. Conclusions and Relevance: Accountable care organizations may be an effective approach to target care among high-risk patients. In this study, physician groups that participated in the MSSP ACO program cared for more clinically vulnerable patients than did nonparticipating groups, and ACO-participating physician groups cared for an equally large number of socially vulnerable patients compared with nonparticipating physician groups.
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