If a person is age 65 or older and they have group health coverage based on their current employment or the employment of their spouse and the employer has twenty or more employees, the group health plan generally pays first.
In addition, Medicare would generally pay second if both of these apply:
A person’s employer joins with other employers or employee organizations (like unions) to sponsor a group health plan (called a multi-employer plan)
and any of the other employers have 20 or more employees
If a person’s group health plan does not pay all of their bill, their doctor or health care provider can send the bill to Medicare for secondary payment.
Medicare will pay based on what the group health plan paid, what the group health plan allowed, and what the doctor or health care provider charged on the claim.
Any costs that Medicare or the group health plan doesn't cover will have to be paid by the insured.
Employers with 20 or more employees must offer current employees 65 and older the same health benefits, under the same conditions, that they offer employees under 65. If the employer offers coverage to spouses, they must offer the same coverage to spouses 65 and older that they offer to spouses under 65.
If the employer has fewer than 20 employees, Medicare generally pays first.
The employer plan could have asked for an exception. So, even if their employer has fewer than 20 employees, the Medicare beneficiary should contact their employer to find out whether Medicare pays first or second.