Physician Competition in the Era of Accountable Care Organizations.

OBJECTIVE: To calculate physician concentration levels for all U.S. markets using detailed data on integration and accountable care organization (ACO) participation.

DATA SOURCE: 2015 SK&A office-based physician survey linked to all commercial and public payer ACOs.

STUDY DESIGN: We construct three separate Herfindahl-Hirschman Index (HHI) measures and plot their distributions. We then investigate how prevailing levels of concentration change when incorporating more detailed organizational features into the HHI measure.

PRINCIPAL FINDINGS: Horizontal and vertical integration strongly influences measures of physician concentration; however, ACOs have limited impact overall. ACOs are often present in competitive markets, and only in a minority of these markets do ACOs substantively increase physician concentration.

CONCLUSIONS: Monitoring ACO effects on physician competition will likely have to proceed on a case-by-case basis.