A Medicare patient is always a Medicare patient, even if they have another payor. When a Medicare patient has been in an auto accident for example, they are still a Medicare patient and all Medicare rules apply.
When billing another payer on this Medicare patient, remember to use the "AT" modifier after the Medicare covered services. This Medicare patient should be signing an ABN each visit as they should be aware Medicare probably will not pay for the covered services of the adjustments, and also non-covered services, like exams and x-rays.
The non-covered services, when billed, will be followed with the "GX" modifier and the Medicare covered services will have both the "AT" and "GA" modifiers, indicating active treatment but not paid by Medicare, so patient signed the ABN. Items #10A thru 10C on the claim form will indicate a PI case or WC case.
When the claim is completed, before mailing to PI insurance company, make two copies. One copy will be your office copy and the second copy will be sent to Medicare indicating to the Medicare Carrier of the PI case with this Medicare patient and telling the Medicare Carrier not to pay.
If you are a non-participating provider in Medicare you must remember not to bill any payer on this Medicare patient above the "limiting" charge Medicare has set for you. Mandatory Claim Submission says you must bill Medicare for covered services within one year of services.
If your patient for some reason loses the PI case, now Medicare should be notified and Medicare will pay to you 80% of the covered services that you have already billed. The only time Medicare should pay on a PI case is if the case has been lost. If Medicare pays on a PI or WC case to the doctor, when the case is settled the doctor must refund that in which Medicare paid, or the Medicare Carrier will take whatever was paid from the doctor's Social Security Retirement Fund.