Medicare as a Secondary Payer
There are many exclusions in which Medicare will not be a secondary payer. The law is found at 42 U.S.C.§ 1395(y). The language is somewhat dense, but If you have Part A or Part B coverage, as a general observation, the secondary payer provisions work like this:
Primary payers are those that have the primary responsibility for paying a claim. Medicare remains the primary payer for those who are not covered by other health insurance. However, in certain conditions are met, Medicare may remain the primary payer. The general rules for most people over the age of 65:
The individual has group health insurance, but that person's employer has less than 20 employees, then Medicare is the primary payer. If the employer has more than 20 employees, then the group health insurance is the primary payer.
Where the individual is over 65, is retired, and has group health insurance through a retirement plan, Medicare is the primary payer.
Where an individual is covered by TRICARE, Medicare is the primary payer. TRICARE includes covering Medicare's co-insurance and deductibles.
Please note that the above is only a quick summary. There are numeroous exceptions and divergences in both Medicare, Disability, Worker's Compensation and TRICARE.