Dear Liz: My credit score just dropped more than 100 points within 45 days. The only thing I can think of that might have caused it is a $46 medical bill that was paid by my flexible spending account. I have a confirmation that the bill was paid, but for some reason the bill went to a collection agency. How do I get my credit score back to 828? I just recently moved and need a good credit rating for numerous reasons, especially purchasing a home and a new car. I was just turned down for a credit card from the bank that holds my mortgage. I tried dealing with the original medical office that received my payment, but they said I have to talk to the collection agency.

Answer: Check first to see if the collection account is actually on your credit reports. Go to http://www.annualcreditreport.com, the only site that offers you free, federally mandated annual access to your credit files at the three major credit bureaus. Other sites may advertise “free” credit reports, but they often come with strings attached such as requirements that you sign up for credit monitoring. Sites that offer free scores typically aren’t providing the FICO scores that most lenders use.

If the collection account isn’t on your reports, something else may have caused the score plunge. Consider buying at least one of your FICO scores from MyFico.com, which will give you an explanation of why your score isn’t higher.

If you find the collection account on your records, however, you need to go back to the medical billing office and insist that someone fix this, said Gerri Detweiler, a credit expert for Credit.com.

“The bill did not magically turn up in collections,” Detweiler said. “Someone made a mistake and since it is their office that was the source of the mistake, they need to fix it.”

Detweiler recommends sending a certified letter explaining that the office has damaged your credit reports and that if someone doesn’t fix the mistake immediately, you will be talking to an attorney about a credit damage lawsuit.

“If the medical office placed it for collections, they can pull it back from collections,” Detweiler said. “It sounds like they are being lazy by refusing to help.”

If the office balks for any reason, you can follow up with an attorney (you can get referrals from the National Assn. of Consumer Advocates at http://www.naca.net). You also can send a certified letter to the collection agency explaining the mistake and insisting it be removed from your credit reports.

You should mention in the letter that you’re trying to get a mortgage and a car loan and that if you’re unsuccessful because of this error, you’ll be talking with a consumer law attorney. It would be helpful to include proof of the mistake, Detweiler said. In many cases, the collection agency will simply delete the erroneous information rather than face getting sued.

“They may not want to bother with it since it’s such a small amount and not worth risking a lawsuit over,” Detweiler said.