Part B Claims Processing

I have a client who has Aetna as her supplement. She wants to see a new doctor and they are telling her they will not accept her Medicare Supplement because they do not have a contract with Aetna. To no avail I have talked with the office manager and explained Medicare is primary and that Aetna is her Medicare Supplement. I can't fix stupid, but if my client goes to this doctor and they just file with Medicare will Aetna automatically receive the claim once Medicare approves the claim? Does the doctor's office have to file with both Medicare and Aetna in order for Aetna to pay the 20%.

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