If a Medicare Beneficiary has Medicare and other health insurance or coverage, each type of coverage is called a "payer." When there's more than one payer, "coordination of benefits" rules decide which one pays first. The "primary payer" pays what it owes on their bills first, and then sends the rest to the "secondary payer" to pay. In some cases, there may also be a third payer.
What do primary and secondary payers pay on a bill?
The insurance that pays first referred to as the (primary payer) pays up to the limits of its coverage.
The insurance that pays second referred to as the (secondary payer) only pays if there are costs the primary insurer didn't cover.
The secondary payer (which may be Medicare) may not pay all the uncovered costs.
If their employer insurance is the secondary payer, they may need to enroll in Medicare Part B before their insurance will pay.
If the insurance company doesn't pay the claim promptly (usually within 120 days), their doctor or other provider may bill Medicare. Medicare may make a conditional payment to pay the bill, and then later recover any payments the primary payer should've made.