Coordination of benefits (COB)

COB exists because two plans will not both pay in full for the same service. Each payer keeps a record of the other coverage it knows about and applies order-of-benefit rules, set by the plan, the state and federal law, to decide whether it is primary, secondary or further down the line. The rules depend on who the subscriber is, whether the coverage comes from active employment, whether Medicare or Medicaid is involved, and the type of plan. The patient’s coverage record has to agree on every payer’s side; when the records disagree, the claim stops.

On the claim, the primary payer’s adjudication drives everything after it. The secondary needs the primary’s allowed, paid and adjustment amounts to work out what it owes. If the order is wrong, the secondary denies for other coverage, or pays as primary and recoups later when it finds out. COB denials tend to sit, because the payer wants the member, not the provider, to confirm or update the coverage record, and the account ages while that call waits.

The mistake is treating COB as a back-end problem. It is settled at eligibility verification. Ask about other coverage at every visit, read the other-payer segment of the 271 response, and when a payer flags a COB issue, call the patient the same day with exactly what the plan needs them to confirm. A claim billed to the right primary payer the first time never becomes a COB denial.