MORRISON, Colo. — 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.

Swedish Medical Center in Denver, like most hospitals in the state of Colorado, employs independent physicians who often don't accept the same insurance as the hospital. Ray Farmer / NBC News

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."

McGuire, the surgeon who performed Briggs' appendectomy, said he's in-network for more than three quarters of his patients and that his billing company works to help patients secure payment and appeal denials from their insurance companies.

"When an insured patient's bill winds up in the legal system, which only happens one tenth of one percent of the time, that insurance company has put the patient in an impossible position, often by refusing to acknowledge that the surgery was emergency care," he said. "The insurance companies hold the power to resolve these issues, and for whatever reason, they choose not to."

After Briggs told her story on KUSA last November, McGuire instructed his billing company to change its collection practices, stopping all future legal proceedings against insured patients who received care that their insurance company did not cover.

Briggs' insurance case worker, who dealt with her billing issues on behalf of the insurance company after Briggs received the surprise bill, declined to comment.

A shared responsibility

The issue goes beyond one patient receiving a surprise bill from one doctor at one hospital in Denver. Colorado legislators are already working to stop physicians from billing patients who receive care at in-network hospitals that accept their insurance plans.

A lawyer for the state association of collection agencies said in a statement that the trade group and Credit Systems, Inc., the collection firm that pursued payment from Briggs, both "[support] changes in the law now pending in the Colorado General Assembly that would address the issue of these billings."

"These cases are challenging for those tasked with collecting the claims as well," said the statement.

Katherine Mulready, chief strategy officer of the Colorado Hospital Association, recognizes that there are many faults in the system and that hospitals have a shared responsibility to do something about these surprise bills — like telling patients they might be treated by an out-of-network doctor.

"That is absolutely an obligation we are willing to make as hospitals," Mulready said, but it is not the only issue.

"There is a mutual dependence between hospitals and physicians, and not all physicians, but certain types of physicians," Mulready said. "We don't always have the ability to tell physicians how they should run their businesses at the end of the day."

On the national level, there is agreement from all parties involved, including the American Hospital Association and the lead insurance industry trade group, America's Health Insurance Plans, on the need for a federal fix.

And there's widespread bipartisan support to do just that. Three bills are currently circulating in Congress, all aiming to put an end to surprise medical billing through different approaches.

A draft bill in the works would ban balance billing, when a patient is expected to pay the difference between what the doctor charged and what the insurance paid. It calls for the state to set a fixed rate for services provided by doctors.

One bill introduced into the Senate last year calls for limitations on cost-sharing for out-of-network services. That's the share of costs covered by insurance that patients pay out of pocket.

Another bill introduced into the House of Representatives in 2017 would require hospitals to notify patients, at least 24 hours in advance, if the hospital and/or any doctor providing care is not in their insurance network. Hospitals would also have to disclose to patients how much they could expect to pay out-of-pocket.

Until federal legislation is passed, patients are left to stick up for themselves and be their own healthcare advocates. Health policy experts say the best thing a patient can do before receiving care — whether for a planned procedure or in an emergency if possible — is call the insurance company and make sure that the hospital and the doctor are both in-network.

For patients like Briggs, that might not have been enough. Three years later, she's still paying her bill.

"I hopefully can bounce back in the next year or so," Briggs said, "but I'll never forget how horrible this was. I'll never forget how many times I cried to my husband, to my mom, to everybody about how frustrating and how illogical this system is."

She's even testified in the state legislature, arguing to make it illegal for collection agencies to garnish wages for unpaid medical debt, which she says crippled her life.

"They all made money off of me," Briggs said. "They made money off of me during one of the worst days of my life."