• J Michael McWilliams, Michael E Chernew, Jesse B Dalton, and Bruce E Landon.
• From the Department of Health Care Policy, Harvard Medical, Boston, Massachusetts2Division of General Internal Medicine and Primary Care, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts.
• JAMA Intern Med. 2014 Jun 1;174(6):938-45.

ImportanceFostering accountability in the Medicare Accountable Care Organization (ACO) programs may be challenging because traditional Medicare beneficiaries have unrestricted choice of health care providers, are attributed to ACOs based on utilization, and often receive fragmented care.ObjectiveTo measure 3 related constructs relevant to ACO incentives and their capacity to manage care: stability of patient assignment, leakage of outpatient care, and contract penetration.Design, Setting, And ParticipantsUsing 2010-2011 Medicare claims and rosters of physicians in organizations participating in ACO programs, we examined these constructs among 524,246 beneficiaries hypothetically assigned to 145 ACOs prior to the start of the Medicare ACO programs. We compared estimates by patient complexity, ACO size, and the primary care orientation of ACO specialty mix.Main Outcomes And MeasuresThree related construct measurements: stability of assignment, defined as the proportion of patients whose assignment to an ACO in 2010 was unchanged in 2011; leakage of outpatient care, defined as the proportion of office visits for an assigned population that occurred outside of the contracting organization; and contract penetration, defined as the proportion of Medicare outpatient spending billed by an ACO that was devoted to assigned patients.ResultsOf beneficiaries assigned to an ACO in 2010, 80.4% were assigned to the same ACO in 2011. Of those assigned to an ACO in 2010 or 2011, 66.0% were consistently assigned in both years. Unstable assignment was more common among beneficiaries with fewer conditions and office visits but also among those in several high-cost categories, including the highest decile of per-beneficiary spending. Among ACO-assigned beneficiaries, 8.7% of office visits with primary care physicians were provided outside of the assigned ACO, and 66.7% of office visits with specialists were provided outside of the assigned ACO. Leakage of outpatient specialty care was greater for higher-cost beneficiaries and substantial even among specialty-oriented ACOs (54.6% for lowest quartile of primary care orientation). Of Medicare spending on outpatient care billed by ACO physicians, 37.9% was devoted to assigned beneficiaries. This proportion was higher for ACOs with greater primary care orientation (60.0% for highest quartile vs 33.6% for lowest).Conclusions And RelevanceCare patterns among beneficiaries served by ACOs suggest distinct challenges in achieving organizational accountability in Medicare. Continued monitoring of these patterns may be important to determine the regulatory need for enhancing ACOs' incentives and their ability to improve care efficiency.

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