There is a five-digit “CPT” code for every medical procedure that health-care providers perform. If one of these Current Procedural Terminology (CPT) codes is entered incorrectly by your provider on a claim submitted to your insurance company or Medicare, the result could be big out-of-pocket costs for you—and errors such as these are extremely common. One recent study by the US Department of Health & Human Services found that a staggering 42% of Medicare claims for “evaluation and management” were incorrectly coded.

Insurance companies and the Medicare system shoulder much of the cost of miscodings, but patients can face inflated bills, too, in the form of higher co-pays…overcharges that must be paid out of pocket before deductibles are met…or outright denials of coverage because misentered codes refer to procedures deemed not medically necessary.

What to do: When you receive an Explanation of Benefits (EOB) statement from your insurance company or Medicare following medical treatment, read the description listed for each service provided. (If an EOB does not provide descriptions, just a series of five-digit alpha-numeric codes, enter these codes into a website such as HealthcareBlueBook.com to find descriptions. If your EOB provides neither codes nor descriptions, contact your insurer and request the CPT codes—these must be supplied upon request.)

Be especially wary if…

The same code appears more than once. It’s possible that you received the same treatment multiple times…but it also is possible that you have been double-billed.

A straightforward office visit is coded as something more extreme. A “level 5” office visit—with the CPT code 99215—should be listed only if you had a life-threatening condition, for example.

What to do: Enter any code you think is questionable into Medicare’s National Correct Coding Initiative Edit (CMS.gov/NationalCorrectCodInitEd). If a description does not sound like the procedure or treatment you received, call the provider’s office and ask whether the code is accurate. Sometimes office personnel acknowledge miscoding when prompted this way and will make corrections.

If you are told that the original coding is correct but you remain skeptical, send a letter to the provider’s billing department officially disputing the bill. Or you can hire a medical billing advocate to contest the bill for you.