The JAMA study was based on eight years of data at 10 institutions that took part in the Medicare Physician Group Practice Demonstration pilot and showed that coordinated care cut medical spending by 5 percent to 10 percent. The biggest savings were in lower hospital readmissions and in acute care among people eligible for both Medicare and Medicaid.
The federal government says its Medicare ACO plans could save up to $940 million over four years. It set 33 measures related to patient safety, preventive health services, at-risk populations and patient experience.
Insurers say they are seeing financial benefits. Aetna Inc, for instance, said hospital admissions have dropped by up to 45 percent in a small Medicare Advantage program - private insurance for seniors - it ran in Maine.
UnitedHealth aims to more than double by 2017 its pay-for-performance medicine, to $50 billion, from $20 billion now.
"Measuring the impact on patients of the commercials ACOs is something we are all interested in doing," Fisher said. "The plans themselves are pretty confident they are doing both - improving care and lowering costs."