How does Medicare determine which insurance is primary and which is secondary?

Medicare determines the primary and secondary insurance status based on certain rules. Generally, if an individual has both Medicare and another type of insurance, Medicare is primary if the person is aged 65 or older and not working, or under 65 and disabled. Medicare is also primary if the person has End-Stage Renal Disease (ESRD) and is in the first 30 months of dialysis or qualifies for Medicare due to a kidney transplant. If the individual has Medicare due to ESRD and is in the 30th month of dialysis or beyond, Medicare is secondary, and the other insurance is primary. If the person has employer/group health coverage, the rules differ based on the employer size. For small employers, Medicare is usually primary, while for larger employers, Medicare is secondary. Additionally, there are specific guidelines for people with Veterans Affairs (VA) benefits or Federal Black Lung benefits. Ultimately, Medicare determines the insurance order to ensure proper coordination of benefits and cost sharing.