Coordination of benefits (COB) is the set of rules that determine which plan pays first when a patient has more than one coverage. The primary plan pays first; the secondary pays after, based on the primary's remittance. COB errors and outdated COB records drive CO-22 denials.
How the order is decided
- Employer coverage vs. Medicare follows Medicare Secondary Payer (MSP) rules.
- For a dependent child with two parents' plans, the 'birthday rule' often applies.
- Active-employee coverage is typically primary over retiree or COBRA coverage.
Fixing a COB denial
A CO-22 usually means the payer believes another plan is primary, or its COB record is out of date. Confirm the correct order, bill the true primary first, then submit to the secondary with the primary's remittance. Appeal only if this payer is genuinely primary and its record is wrong.
Frequently asked
How is the primary plan decided?
Do I appeal a COB denial or fix it?
Primary sources: 42 U.S.C. 1395y(b) (Medicare Secondary Payer). General information, not legal or medical advice — confirm against the governing rule for the plan type.
When the appeal has to be written, and cited
Upload the denied EOB and Merits returns a complete, citation-verified appeal letter — the clinical argument, the payer's own coverage criteria, and your federal appeal rights — in about a minute.