How it works
The primary plan processes the claim first; the secondary plan then considers the remaining balance using the primary plan’s remittance. Common rules include the “birthday rule” for children covered by both parents (the plan of the parent whose birthday falls earlier in the year is usually primary) and Medicare Secondary Payer rules, under which an employer group plan is primary to Medicare in many situations for patients who are still working.
Why it matters
Billing the wrong plan first leads to denials such as CO 22 (care may be covered by another payer). Plans also periodically ask patients to update their other-coverage information, and claims can be held until they do.
Example
A patient turns 65, enrolls in Medicare and keeps coverage through a large employer while still working. The employer plan is billed first and Medicare second. Registration captures both, in the right order.