Aside from expanding coverage, one of Obamacare's main goals is to better coordinate care, with the idea of lowering costs and producing better health outcomes. A new study hints at challenges that could undermine those efforts.

The Affordable Care Act encourages the formation of Medicare Accountable Care Organizations, which are provider networks that are financially incentivized to keep down costs and keep patients healthier. That means there should be a pretty significant effort across the ACO provider network to ensure that care is being properly coordinated. In short, the theory here is that when doctors talk to each other and share the same goals — which is not always a given — patients get better care and costs are kept down.

One possible problem, according to a new study in JAMA Internal Medicine: Coordination could be difficult because of how fragmented care is in the existing Medicare program.

To make this point, Harvard Medical School researcher J. Michael McWilliams and some colleagues looked at Medicare claims and physician rosters for 2010 and 2011, the two years just before the Medicare ACO program launched. McWilliams studied how Medicare beneficiaries sought out care and whether they were going to providers that would eventually be within the same ACO. (The more patients staying within the ACO, the better chance participating providers will have to meet program goals.) In short, the research essentially imagines the ACOs were in place in 2010 and 2011, using data from 525,000 beneficiaries across 145 provider groups that eventually formed ACOs.

Under this assumption, about 80 percent of beneficiaries would have received the bulk of care from the same providers within the ACO both years. However, beneficiaries were much more likely to stay with the same primary care doctors than the same specialists. Just 8.7 percent went to primary care physicians outside what would have been the ACO network, while about two-thirds had office visits with specialists outside that network. Those more likely to get care outside the ACO network tended to be patients with chronic conditions — who would arguably benefit most from having better coordinated care.

Of course, these results are hypothetical, but McWilliams said it provides a look at challenges ahead for current ACOs and providers thinking about joining the program. Also, the program is still new, with likely tweaks ahead.

Medicare now counts 360 ACOs serving a combined 5.3 million beneficiaries (about 12 percent of the entire program). Medicare recently said the program's first-year savings amounted to $380 million, with about half of the 114 ACOs in the program's first year having lower-than-projected expenditures. Still, parsing out whether ACOs are meeting performance expectations is difficult.

One issue to consider, McWilliams said, is that Medicare beneficiaries have free reign to choose their doctors. Paul Ginsburg, a former longtime president of the Center for Studying Health System Change, said the health-care law doesn’t offer Medicare beneficiaries incentives to stay within the ACO network, and they don't even know in advance whether they're part of the ACO.

“This absence may severely undermine the potential of this approach to improve care and control costs,” Ginsburg wrote in a perspective accompanying McWilliams's study.

McWilliams said ACOs have some tools at their disposal to encourage patients to stay within their provider network, like free screenings or some marketing. A group advising Congress on Medicare issues is already looking at ways to encourage patients to seek care within a single ACO, and other proposals will emerge.

"This is likely to be a moving target, which is why we were interested in doing this," McWilliams said.