Among hospitals accepting bundled payments, simultaneous "co-participation" in accountable care organizations (ACOs) could impact episode outcomes compared to bundled payment participation alone. Difference-in-differences (DID) analysis of 1 857 653 ACO-attributed Medicare beneficiaries. The study exposure was hospitalization for 24 procedure-based and 24 condition-based episodes at hospitals participating in bundled payments and ACOs (co-participant) versus only bundled payments. Study outcomes included episode quality, postacute utilization, and spending. For procedure-based episodes, patients hospitalized at co-participant and bundled payment hospitals did not exhibit differential changes in risk-adjusted mortality (DID 0.04 percentage points [p.p.], 95% confidence interval [CI] -0.28 p.p. to 0.37 p.p., P = 0.79), readmissions (DID -0.32 p.p., 95% CI -1.5 p.p. to 0.82 p.p., P = 0.59), postdischarge institutional spending (DID $119, 95% CI -$216 to $455, P = 0.49), or postacute utilization. Similarly, outcomes for condition-based episodes did not vary between co-participant and bundled payment hospitals. Payment model co-participation may produce neither synergistic benefits nor negative effects for patients.
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