Treating a sprain in a U.S. hospital can cost anywhere from $4 to $24,000, making a trip to the emergency room financially risky for uninsured patients, researchers said.

A study found wide variety between the lowest and highest charges for 10 common conditions, highlighting the unpredictability of health-care costs. Bills ranged from $50 to more than $73,000 for urinary tract infections, $15 to $17,797 for headache care and $29 to $29,551 for intestinal infections.

“There is a huge amount of variation in the prices charged for medical care and, unfortunately, our system is set up in a way where patients really can’t do much about it,” said Renee Hsia, an assistant professor of emergency medicine at the University of California, San Francisco, and author of the study, published last week in the journal PLoS One.

While these prices are what hospitals bill for their services, what a patient ends up paying depends on whether they have insurance and what kind, Hsia said. Larger insurance companies can negotiate lower rates, while the uninsured usually have to pay the entire bill, she said.

“They have no one to negotiate on their behalf, and they are stuck with the bill,” Hsia said. “For privately insured patients, if they accidentally end up in an out-of-network hospital, or their annual limit has been reached, then they, too, are faced with the entire bill.”

The study examined emergency room visits by 8,303 adults in 2006 to 2008 that didn’t involve admittance to the hospitals. The median charge was $1,233 for an ER visit, more than the average American’s rent, Hsia said.

The researchers excluded patients 65 or older, those covered by Medicare and people who left with more than one diagnosis, in order to narrow the study. The data came from the U.S. Agency for Healthcare Research and Quality’s Medical Expenditure Panel Survey.

The numbers themselves can seem arbitrary, said Giovanni Colella, a doctor and co-founder of Castlight Health, a San Francisco company that helps patients shop for medical care.

“I’ve been in this business for 25 years, and I still can’t figure out how they determine these prices,” Colella said. “The paradox of our health-care system is that if you’re uninsured, you’re going to pay much higher amounts.”

There’s often no way for patients, or doctors, to know what the final charge will be, Hsia said. A person could fall off a ladder, or a one-story building, and come to the hospital with a swollen ankle. Doctors may order X-rays, and if they came back negative, further tests such as CT scans might be needed to determine whether there’s a fracture, Hsia said.

Medical spending in the U.S. increased 3.9 percent in 2011 to $2.7 trillion, according to the latest figures from the Centers for Medicare and Medicaid Services, and is expected to rise 3.8 percent this year. Out-of-pocket health costs, which were $304.4 billion in 2011, should rise 3.4 percent this year, the agency said in a report.

“To a hospital, a patient may be an object of human compassion, but he or she is also a biological structure yielding cash,” said Uwe Reinhardt, a professor of economics at Princeton University, in an e-mail.

“In general, a hospital will try to extract as much cash from that structure as it can, and that depends on the bargaining power of whoever pays for the patients.”