NEW YORK (MONEY Magazine) - Q. Last year my wife thought she was having a heart attack, so we called 911. Luckily, it was just a scare. But the hospital keeps sending us a bill for $900. I was assured that all her bills would be covered 100 percent. We have Medicare, plus my retiree health insurance, with all the bells and whistles. Now the hospital is threatening to turn the case over to a collection agency. Help!

Robert Ottman, Wilmington, N.C.

A. Welcome to the chaotic world of hospital billing. Thanks to inexplicable pricing codes, impenetrable rules and confusing insurance requirements, nearly eight out of 10 hospital bills contain multiple errors, according to the Medical Billing Advocates of America.

We asked Pat Pane, a certified medical-claims assistant in your hometown of Wilmington, to walk us through a solution. (A medical-claims assistant is a relatively new type of expert trained to decode bills and troubleshoot for frustrated consumers.)

First she reviewed the itemized hospital bill that should have been sent to you (but wasn't) and to your two insurers, Empire Blue Cross/Blue Shield and United Healthcare.

After cross-referencing the documents and reviewing your policies, she spotted the problem: The hospital had neglected to send the ambulance portion of your bill to the proper insurer.

"I see this type of thing from clients around the country," says Pane. In your case, hospital bills are covered by Medicare Part A and get sent to Blue Cross; ambulance bills are covered by Medicare Part B, which should have gone to United Healthcare.

Pane had the hospital resubmit the bill to the correct insurer. As we went to press, a payment had been sent to your hospital.

Nifty, huh? Medical-claims assistants charge $30 to $150 an hour. To find one near you, go to claims.org or billadvocates.com.

One more thing: Check your credit report by year-end. Zealous billing departments have a way of keeping these nightmares alive by reporting disputes as delinquencies. Examine your reports online at myfico.com; for $38.95 you can buy yourself some peace of mind.

Got a question about money? Have a financial or red-tape nightmare? Need an advocate or some good advice?

E-mail us at money_helps@moneymail.com.