Compared with non-expansion states, the share of emergency department visits covered by Medicaid increased 8.8 percentage points in expansion states — whereas the uninsured share decreased by 5.3 percentage points.

While Democrats lobbied to pass the ACA in part by claiming more insured people on the rolls would result in an overall reduction in emergency department visits, increased visits may not necessarily be a bad thing, writes Ari Friedman in an editorial accompanying the research study.

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"Much of the policy literature to date would view these additional ED visits as a failure of the health system," writes Friedman, a post-doctoral resident at Beth Israel Deaconess Medical Center in Boston, Mass. "To form an opinion on whether the system is harmed by these visits, however, we need to know the cost of ED visits and of their alternatives, as well as the benefits of each."

For example, a person with pharyngitis visits an emergency room, receives brief reassurance and is discharged, for a price of about $700, he writes. That cost could likely be the same at a doctor's office. However, emergency department visits on average tend to have higher mortality rates for the same diagnosis than clinic visits, so perhaps visiting a primary care clinic may be better for continuity of care — provided that coordination is done well.

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"Any scheme to divert patients from the ED must wrestle with these three parameters: the cost of an ED visit, the cost of alternatives per ED visit reduced, and the number of patients who will be harmed through this diversion," Friedman writes.

"In this view, reducing ED visits is an intervention like any other, in which these three numbers can be used to calculate the cost-effectiveness of diverting ED visits. Nikpay et al and others have now shown through careful empirical work that Medicaid expansion does not decrease and may increase the number of ED visits. It will take equally studious investigation to move beyond the absolutes and value judgments that have dominated the study of ED utilization toward a more rational basis for how we structure unscheduled visits in the health system."

Meanwhile, a study published by the Urban Institute concludes as many as 14.8 million Americans could lose Medicaid coverage by 2022 if all states drop ACA expansions and cut more enrollments to account for per capita caps and other federal funding cuts.

"Many states may have no choice but to eliminate coverage of the expansion population because they would be unable to substantially increase their own spending," the authors write.

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"Moreover, they have limited scope to cut benefits and provider payment rates. Cuts to benefits such as dental, vision, and hearing coverage do not yield much savings. Provider payment rates are already very low in most states. Thus, the likelihood of enrollment cuts seems high, particularly among low-income states."

The study also finds federal Medicaid spending could decrease by 20.5 percent, or $938.3 billion, from 2019 to 2028, while state spending might go down by $78 billion over the same time.