(HealthDay News) — There are significant challenges to achieving organizational accountability in accountable care organizations (ACOs) assigned to Medicare beneficiaries, according to a study published online April 21 in JAMA Internal Medicine.

J. Michael McWilliams, MD, PhD, from Harvard Medical School in Boston, and colleagues examined three constructs pertinent to ACO incentives (stability of patient assignment, leakage of outpatient care, and contract penetration) based on rosters of physicians in organizations participating in ACO programs and Medicare claims (2010 to 2011).

The researchers found that 80.4% of the beneficiaries assigned to an ACO in 2010 were assigned to the same ACO in 2011. Beneficiaries with fewer conditions and office visits as well as those with the highest decile of per-beneficiary spending were more commonly associated with unstable assignment. 

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Nearly 9% of ACO-assigned beneficiary office visits with primary care physicians were provided outside of the assigned ACO, as were 66.7% of office visits with specialists. Higher-cost beneficiaries had greater leakage of outpatient specialty care, which was substantial even among specialty-oriented ACOs. 

Medicare spending on outpatient care billed by ACO physicians was higher for ACOs with greater primary care orientation.

“Continued monitoring of these patterns may be important to determine the regulatory need for enhancing ACOs’ incentives and their ability to improve care efficiency,” the researchers wrote.

In an invited commentary, Paul B. Ginsburg, PhD, of the Sol Price School of Public Policy, University of Southern California, Los Angeles, commented on what is necessary for future success.

“By creating a formal and mutually acknowledged relationship between ACOs and beneficiaries, health care provider organizations that make the investments needed to coordinate care, manage chronic diseases, and manage population health would be more likely to succeed,” Ginsburg wrote.


  1. McWilliams JM, Chernew ME, Dalton JB, et al. Outpatient Care Patterns and Organizational Accountability in Medicare. JAMA Intern Med. 2014;doi:10.1001/jamainternmed.2014.1073.
  2. Ginsburg PB. Accountable Care Organizations 2.0: Linking Beneficiaries. JAMA Intern Med. 2014;doi:10.1001/jamainternmed.2014.161.