When Nicole Briggs felt intense stomach pain one night three years ago, she went to a freestanding ER near her home in the Denver suburbs. She was diagnosed with appendicitis and told she needed surgery as soon as possible.

She rushed to a nearby hospital, Swedish Medical Center — but first called ahead to make sure it took her insurance.

When the hospital said yes, Briggs thought that meant she was covered. "I thought [that meant] the anesthesiologist takes my insurance," she said, "that the surgeon [does], that the nurse, you know, that that's all part of the same deal."

But two months after the surgery, she got a whopping bill for $4,727 from the surgeon, Dr. Emmett McGuire. Like most of the doctors at the hospital, McGuire practiced independently. He did not take her insurance.

She declined to pay the bill. Two years later, a collection agency slapped a lien on her home, which would block her ability to sell her house until she paid off the debt.

"It's really scary," Briggs said. "This is all we have, and to think that it could all be taken away, because some doctor doesn't feel like taking anyone's insurance is — it's just so wrong."

She isn't the only one facing this "scary" reality. NBC's Denver affiliate KUSA worked for months on a series about surprise medical bills, and found that since 2017, just one collection agency had put liens on 170 local homes. When NBC News contacted Credit Systems, Inc., the company declined to comment.

NBC News dug further and found similar cases of liens on homes because of unpaid medical bills in at least five other states: New Hampshire, Nevada, Ohio, Oklahoma and Vermont.

Surprise medical bills like the one Briggs received in the mail can easily lead to these liens. A recent poll by Kaiser Family Foundation found that four in 10 Americans have received a surprise medical bill.

"I find this really unconscionable," said Dr. Ashish Jha, a health policy professor at Harvard's Chan School of Public Health. "This is really a failure of our system to stick people with these kinds of bills that really have no justification whatsoever."

Briggs says that when she got to Swedish Hospital Center for her surgery three years ago, she asked again and again to be sure she was fully covered. Even after she was told the hospital took her insurance, she says she kept asking — more than a dozen times.

What she didn't ask explicitly is whether the surgeon was covered.

When she got her surprise bill for $4,727, she says she called her insurance company to explain the situation. She hoped it would pay the bill, but the company told her she was on the hook for the full amount and would have to pay it.

"I was so frustrated with it," Briggs recalled. "I thought, 'I'm just going to let this die in collections. I'm not going to pay this.'"

But the bill didn't die. Instead it kept collecting interest, growing to $5,802.08. Two years later, a collection agency called Credit Systems, Inc. took her to court representing the surgeon. The company sued her for the new amount and won.

Then came a letter in the mail announcing the lien on her home. Soon after, the collection agency started garnishing her wages by 25 percent every month, forcing her to pay it off. Pregnant and about to go on maternity leave, she worried she wouldn't be able to afford the pay cut.

Several health policy experts agreed that all parties share blame in the system: the doctors, insurance companies and hospitals. Doctors can charge much higher prices for out-of-network care if they don't accept insurance. Insurance companies might negotiate service rates that are not competitive enough for doctors and hospitals to conduct their business. That could lead doctors to charge patients those out-of-network rates.

"Patients have no way of knowing this upfront before it happens," said Jha. "Physician groups see this as a way to make extra money on the backs of patients and I think that's a very serious problem."

Hospitals are on the hook as well. They often contract out-of-network doctors, especially for emergency care providers like surgeons, anesthesiologists and radiologists who are needed around the clock. Plus, hospitals are not required to track providers' insurance networks.

But they know doctors are charging out-of-network, said Jha.

"Hospitals have an obligation to make sure that every doctor who works within their walls is generating bills that either are part of the insurance that patients have or are things that patients can afford," Jha said.

In Briggs's case, Swedish Medical Center, the hospital where she was treated, said the vast majority of its physicians practice independently, which is true of most hospitals in Colorado and many others across the country.

Swedish Medical Center said in a statement that it cannot compel these independent doctors to accept the same insurance plans that the hospital does. "We strongly encourage those independent physicians to participate in the same insurance networks as we do; however, ultimately those negotiations occur between the individual providers and insurers, not hospitals."

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