COB denials occur when a patient has more than one insurance plan and there is a dispute or confusion about which plan is primary. The denial tells you the claim was submitted to the wrong payer first, or that COB information is missing or conflicting.
Coordination of Benefits denials are among the most solvable in medical billing, once you understand who is primary and why. The resolution path is almost always to submit to the correct payer in the correct order, or to provide the COB information the payer needs to adjudicate.
How COB Works
When a patient has more than one health insurance plan, one is designated primary (pays first) and one is secondary (pays the remaining balance after primary adjudication). The rules for determining primary vs. secondary coverage depend on the type of plans involved:
- Birthday rule (most commercial plans): The parent whose birthday falls earliest in the calendar year is primary for dependent children
- Medicare as secondary: Medicare is secondary to employer-sponsored insurance for active employees, Workers' Compensation, and auto/liability insurance
- Medicaid is always last: Medicaid is payer of last resort, always secondary to any other coverage
- Active vs. retired status: Coverage from active employment is typically primary over retiree coverage
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