CHICAGO -- Here's an added challenge if you're struggling to pay a medical bill: Your credit can be wrecked if the doctor or hospital kicks your unpaid bill to a collection agency.

A growing number of Americans are discovering this unexpected land mine when they refinance or take out a loan. The Commonwealth Fund, a private foundation that sponsors health care research, estimates that 22 million Americans were contacted by collection agencies for unpaid medical bills in 2005. That increased to 30 million in 2010.

People with wrecked credit scores include those who were just trying to resolve a dispute with their insurance company. It includes those owing less than $250. Even after the bill is paid, the record of the collection action can stay on a credit report for up to seven years.

Mike and Laura Park thought their credit record was spotless. The couple wanted to take advantage of low-interest rates, so they put their house on the market and talked to a lender about a mortgage on a bigger home in the Dallas-Fort Worth suburbs.

Their credit report contained a shocker: A $200 medical bill sent to a collection agency, but since paid, lowered their credit scores by about 100 points, and they'll have to pay a discount point to get the best interest rate. Cost to them: $2,500.

An estimated 3.4 million Americans have paid-off medical debt lingering on their credit reports, according to the Access Project, a research group funded by health care foundations and advocates of tougher laws on medical debt collectors.

Nathen and Melissa Cobb of Riverton, Ill., tried to refinance their home. They didn't qualify for the loan because of $740 in medical bills sent to a collection agency. They were surprised because the bills, nearly a dozen small co-payments ranging from $6 to $280, had been paid before they tried to refinance.

"I'm not one of those people trying to ditch out on my bills," Melissa Cobb, 34, said. "I'm really frustrated."

The Parks had no idea a billing error they'd sorted out a year earlier -- they never actually owed the $200 -- could affect their credit.

"We've prided ourselves in having impeccable credit. We worked hard to establish that," said Laura Park, 51, an office manager married to a 53-year-old firefighter.

"I'm very upset," Park said. "It's going to be a nightmare and who knows how long this is going to take to resolve?"

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