Adults who are covered by Medicaid use emergency rooms 40 percent more than those in similar circumstances who do not have health insurance, according to a unique new study that sheds empirical light on the inner workings of health care in the U.S.

The study takes advantage of Oregon’s recent use of a lottery to assign access to Medicaid, the government-backed health-care plan for low-income Americans, to certain uninsured adults. The research examines emergency room records for roughly 25,000 people over 18 months.

“When you cover the uninsured, emergency room use goes up by a large magnitude,” says Amy Finkelstein, the Ford Professor of Economics at MIT and a principal investigator of the study, along with Katherine Baicker, professor of health economics at Harvard School of Public Health.

The study, published January 2, 2014 in the journal Science, also documents that having Medicaid consistently increases visits to the emergency room across a range of demographic groups, types of visits, and medical conditions, including types of conditions that may be most readily treatable in primary-care situations.

“In no case were we able to find any subpopulations, or type of conditions, for which Medicaid caused a significant decrease in emergency department use,” Finkelstein adds. “Although one always needs to be careful generalizing to other settings, these results suggest that other Medicaid expansions are unlikely to decrease emergency room use.”

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