Coordination of benefits (COB)
COB is the set of rules deciding which plan pays first when a patient has more than one — and a denial reason when the order is billed wrongly.
Updated
Coordination of benefits (COB) is the process that determines the order in which plans pay when a patient is covered by more than one. The plan that pays first is primary; the plan that pays next is secondary, and it considers the balance remaining after the primary's payment and adjustments.
The order is set by rule, not by preference. Which plan is primary depends on the plans involved and the patient's circumstances, and neither the patient nor the provider chooses it. Nor does the payer: Medicare’s rule is that its “benefits are secondary to benefits payable by a primary payer even if State law or the primary payer states that its benefits are secondary to Medicare benefits or otherwise limits its payments to Medicare beneficiaries” — so a plan asserting it pays second does not make it so.
In practice
COB denials have a structural cause: the payer, not the practice, holds the record of the patient's other coverage, and that record can be out of date or simply unset. A claim billed to the wrong plan first is denied even though the service, the coding, and the documentation are all correct.
Resolving a COB denial is often a patient-contact task rather than a billing one — the patient's coverage record with the payer has to be updated before the claim can be reprocessed.
Commonly confused with
- Eligibility verification: Eligibility asks whether a plan covers the patient; COB asks which of several covering plans pays first.