Average out-of-pocket costs for hospitalization went up for people in the U.S. with insurance between 2009 and 2013, due largely to increasing deductibles and coinsurance, according to a new study.

"These people have pretty good health insurance and we wanted to see how much they'd been spending in out of pocket costs," said lead author Emily R. Adrion of the University of Michigan Medical School in Ann Arbor.

Surprisingly, even people with high quality health insurance plans were still spending a lot out-of-pocket for hospitalization in 2013, she told Reuters Health by phone.

Although the health insurance exchanges under the Affordable Care Act were not implemented until 2014, previous trends in employer sponsored insurance plans haven't changed a lot since then, Adrion said.

The researchers used commercial claims data from 2009 to 2013 for 7.3 million hospitalizations of adults ages 18 to 64. The study subjects were covered by Aetna, UnitedHealthcare and Humana under employer sponsored or individually purchased plans.

Average out-of-pocket costs per hospitalization went from $738 in 2009 to $1,013 in 2013.

Deductibles - the amount an individual must pay for care before an insurer will pay any expenses - rose from an average of $145 in 2009 to $270 in 2013. After paying the deductible, a patient often still has to pay for some percentage of the cost of care, and the employer or insurance company pays the remainder. Coinsurance grew from $518 in 2009 to $688 in 2013, as reported in JAMA Internal Medicine.

More than 80 percent of commercial health insurance benefit packages require coinsurance for inpatient hospitalization in addition to annual deductibles, the authors write.

Coinsurance accounts for a percentage of care expenses, whereas a copayment is a fixed amount. The increase in out-of-pocket costs through 2013 nay have been due to a move from fixed copayments to coinsurance, which can range to much higher amounts for more expensive services, the authors write.

"Insurance provides a lot of protection," and coverage is better than no coverage, Adrion said. If you are injured, your insurer will cover it, "but maybe not quite as much as you might think."

She and her coauthors only had data on patients' age and gender, not on socioeconomic status, so they couldn't say how difficult it was for the patients to pay out-of-pocket costs.

"Definitely this needs to be more a part of policy discussions," Adrion said. "We tend to focus on the uninsured and underinsured and sort of ignore the burden on people with good coverage."

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Extending some of the protections available through the ACA to employer sponsored plans could be helpful, she said.

People don't have a good understanding of what coinsurance is, she said. She advises people to educating themselves about the details of their plans.

In a comment published with the study Dr. Mitchell H. Katz, deputy editor of the journal, notes that in 2013, average out-of-pocket costs were $1,509 for patients hospitalized with appendicitis. People can't do anything to prevent appendicitis and are not in a position to shop for the best price of care when they have that kind of acute condition, he points out.

"There are no easy answers for how to deal with the rising cost of medical care, but increasing out-of-pocket spending for unavoidable, necessary care is counter to the goals of a health insurance system," he writes.