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If one of the tens of thousands of new Medicaid patients in Colorado feels the flu coming on this winter, they'll have a choice in how to get help: They can get treated within a few hours and pay nothing. Or they can make an appointment, wait a few weeks and then pay a $20 co-pay.

It's likely that a lot of these patients will choose the first option, which means going to an emergency room, where they can get care for free under Medicaid.

That's just what many patients in Oregon did when that state expanded Medicaid in 2008. And while it makes economic sense, it flies in the face of what some proponents of the Affordable Care Act have said would happen: that getting more people on health insurance will cut down on costly visits to the emergency room

Katherine Baicker is a researcher at the Harvard School of Public Health and one of the lead authors of a recent study about what happened in Oregon. She says her findings surprised some people. 

"But I think if you step back and think about the incentives for patients, it might not be so clear that you'd expect them to substitute the doctor's office," Baicker says.

"Patients who were hesitant to go to the emergency department when they were uninsured wouldn't face that cost barrier once they gained access to Medicaid and that would suggest an increase to emergency department use, which is just what we saw," Baicker continues.

Baicker and her colleagues also saw increases in use of primary care and other doctors offices as well as increase in prescription drug use.  And she says more people reported having access to a regular doctor's office other than the emergency room.

Judy Zerzan, chief medical officer at Colorado’s Department of Health Care Policy and Financing, says she wasn't surprised to see Baicker's study.

"There's a lot of pent-up demand; this population has a lot of mental and physical health issues," Zerzan says. "It can be hard to find a primary care provider if you don't know where to look."

While Zerzan expects a small increase in the number of ER visits in Colorado due to the Medicaid expansion, she thinks the state is prepared because of the Accountable Care Collaboratives that were established here in 2011.

"That's a program whose main design is to improve people's costs and reduce the cost of Medicaid," Zerzan says.

Through the collaboratives, care coordinators are responsible for getting patients to the right doctors at the right time, which Zerzan says will lessen the likelihood that patients will always consider the ER the best option.

"We also have a nurse advice line in Medicaid," Zerzan says. "And they can give you advice about whether it's best to make an appointment or best to go to the ER."

One strategy that Colorado could consider is to put a co-payment on visits to the ER that doctors determine are not true emergencies. Other states are trying it but Zerzan says results are mixed. 

"It's not clear that that really helps decrease non-emergent use, and in some cases it might decrease emergent use, which can really get people into trouble," Zerzan says.

Governor John Hickenlooper's promise that expanding Medicaid will cut down on health care costs overall in Colorado in the long-term still holds true, says Zerzan, despite the findings in Oregon.

"There's other studies, like on Medicare,” Zerzan says. “People are much more expensive their first year or maybe two on Medicare, often because they haven't had health insurance before. And so they've put off a lot of those things. And then after that first year or two, their health expenditures go down, and we expect the same."