Coordination of Benefits (COB)
Coordination of benefits (COB) is the process of determining which insurance pays first when a patient has more than one plan — primary pays under its normal rules, secondary considers the remainder. COB errors drive CO-22 denials and are among the most preventable denial categories in medical billing.
- Decides
- Which plan is primary, secondary, tertiary
- Key denial
- CO-22 (covered by another payer)
- Dependent children
- Birthday rule (earlier birth month = primary)
- Medicare
- MSP rules decide when Medicare is secondary
How does coordination of benefits decide who pays first?
When a patient has two or more plans, COB assigns an order: primary adjudicates first under its normal rules, then the secondary receives the claim with the primary's payment details attached and considers what is left. Get the order wrong and the claim bounces — typically as a CO-22 denial — and nothing pays until the order is corrected. COB exists so that combined payments never exceed the total charge; it is not a way to get paid twice.
What are the main ordering rules?
- Own coverage beats dependent coverage. A patient's employer plan is primary over a spouse's plan that covers them as a dependent.
- Birthday rule for kids: the parent with the earlier birthday in the calendar year carries the primary plan for dependent children.
- Active employment beats retiree/COBRA coverage.
- Medicare Secondary Payer rules govern when Medicare steps back — working aged with employer group health plans (20+ employees), certain disability and ESRD situations, and any accident-related care where liability, no-fault, or workers comp is involved.
- Medicaid is always the payer of last resort.
What does a two-payer claim look like in dollars?
Office visit billed at $180. Primary (commercial) allows $92.40, applies 20% coinsurance, and pays $73.92, leaving $18.48 patient responsibility. You then submit to the secondary with the primary EOB. If the secondary covers coinsurance and its own allowed amount is at least the primary's, it pays the $18.48 and the patient owes nothing. The secondary's ERA shows the primary's payment offset as OA-23 (impact of prior payer adjudication) — that is bookkeeping, not a denial.
If the secondary's allowed amount is lower than the primary paid, it may pay $0 under non-duplication rules. That is also normal, and it is why "bill the secondary" is not a guarantee of extra money.
How do you prevent COB denials?
- Ask about other coverage at every visit, not just annually: "Any other insurance, including through a spouse? Any recent job changes? Is this visit related to an accident?" Those three questions prevent most CO-22s.
- Run eligibility and read the COB segment. Many 271 responses flag other coverage the patient forgot to mention.
- When CO-22 hits, script the patient call. The payer will only update COB from the member. Give the patient the member services number from your payer phone directory, tell them to say "I need to update my coordination of benefits," and diary the claim for rebill in 2–3 weeks.
- Complete the MSP questionnaire for every Medicare patient and re-verify at least annually.
Frequently asked questions
For a dependent child covered under both parents' plans, the plan of the parent whose birthday falls earlier in the calendar year (month and day, not age) is primary. If both parents share a birthday, the plan that has covered the parent longer is primary. Divorce decrees and court orders can override the birthday rule.
CO-22 means the payer believes another insurer is primary for this claim. Either the patient has other coverage you did not bill first, or the payer's COB file is stale and needs the member to update it. The fix is usually the patient calling their plan to complete a COB questionnaire — the provider cannot do it for them.
Under Medicare Secondary Payer (MSP) rules: working aged with employer group coverage from 20+ employee firms, disability with large group health plans (100+), ESRD during the 30-month coordination period, plus workers comp, liability, and no-fault situations. Ask the MSP questionnaire questions at intake — billing Medicare primary when it is secondary creates recoupments later.
Sources & further reading
Reviewed by the ImmediCare Solutions RCM team
Certified billers and coders handling claims across 50+ specialties nationwide. This entry is reviewed against current payer policy and CMS rules. Last review: Jul 5, 2026.
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