From the Medicare Rights Center
If Original Medicare will not pay for care you received, you will find this out when you receive your Medicare Summary Notice (MSN). If you think the care you received is medically necessary, you should not take no for an answer.
1. Find out if it is possible that there was a billing mistake.
Medicare uses a set of service codes, called CPT codes, for processing medical claims. Each medical service has been assigned a specific code. Sometimes providers accidentally use the wrong codes when filling out Medicare paperwork, and this can result in Medicare denials. A denial can sometimes be easily resolved by asking your doctor to double-check that your claim was submitted with the correct codes. Your doctor's billing office can call 800-MEDICARE to get in touch with the company that processes Medicare claims (carrier or intermediary). If the wrong code was used, ask your doctor to resubmit the claim with the correct code.
2. If the provider believes that the claim was correctly coded or is unwilling to refile the claim, your next step is to appeal.
Appealing is easy and many people win.
The MSN will have instructions for how to appeal. Follow these instructions. If the MSN lists several items and you are not disputing all of them, circle the one you want to appeal. Write "Please Review" on the bottom and sign the back. Make a copy for your files. Then mail the signed original to Medicare at the address on the MSN. Make sure you mail your appeal within 120 days of receiving the MSN.
If possible, get a letter from your health care provider saying that you needed the service and why. Send this with your MSN.
Keep photocopies and records of all communication, whether written or oral, with Medicare concerning your denial. Send your appeal certified mail or delivery confirmation.
Even if you sign an Advance Beneficiary Notice (ABN) that stated that you agree to pay for care if Medicare will not, you can still appeal.
Note: You can not appeal to Medicare to cover services that are never covered. For example, you can never ask Medicare to cover more than 100 days in a skilled nursing facility.