Coordination of benefits (COB) is the set of rules that decides which health plan pays first when a patient has more than one coverage. The primary payer pays up to its limits, and the secondary payer then considers what is left under its own terms and may not pay the full remaining balance. Payer order comes from established rules: employee coverage generally comes before dependent coverage, the birthday rule applies to children, and Medicare Secondary Payer (MSP) rules apply when Medicare is involved. Most COB problems start at registration, when payer order isn’t confirmed before the claim goes out.
How Does Coordination of Benefits Work?
CMS’s January 2026 guide explains that the primary payer pays up to the limits of its coverage, and the secondary payer only pays if there are additional costs the first payer didn’t cover. The secondary payer, which could be Medicare, might not pay all of the remaining uncovered costs. For commercial plans, the NAIC model regulation says a secondary plan calculates what it would have paid as primary and may reduce its payment so combined payments don’t exceed 100 percent of the allowable expense.
Which Insurance Is Primary?
Most commercial plans follow rules based on the NAIC Coordination of Benefits Model Regulation, which many states have adopted. Each plan applies the first rule that fits:
- Employee before dependent: the plan covering a person as an employee, subscriber, or retiree pays before the plan covering them as a dependent.
- Dependent children: the birthday rule or a court decree decides.
- Active before retired or laid off: coverage through active employment pays before retiree or laid-off coverage.
- COBRA: continuation coverage pays after the person’s other plan.
- Length of coverage: the plan that has covered the person longer pays first. If no rule decides, allowable expenses are shared equally.
The birthday rule: when parents are married or living together, the plan of the parent whose birthday falls earlier in the calendar year is primary, using month and day only. If birthdays match, the plan that has covered a parent longer is primary. For separated or divorced parents, a court decree that assigns responsibility comes first, provided the plan knows of it. With no decree, the custodial parent’s plan is first in the order.
The model is not federal law, and self-funded employer plans are generally governed by ERISA rather than state insurance law, so verify payer order with the payers.
Medicare Secondary Payer Rules
Medicare follows federal rules. According to CMS’s MSP Manual and its MLN booklet:
- Age 65 or older, with current employment coverage: the group health plan (GHP) pays first if the employer has 20 or more employees. Otherwise, Medicare pays first.
- Disability, with current employment coverage (own or a family member’s): the GHP pays first if the employer has 100 or more employees. Otherwise, Medicare pays first.
- Multi-employer plans: only one employer needs to meet the threshold.
- ESRD: a GHP generally pays first for the first 30 months, and Medicare pays first afterward.
- Retiree coverage: Medicare pays first.
CMS also states that Medicaid never pays before Medicare, and that no-fault, liability, and workers’ compensation insurance usually pay first for related care. CMS notes its charts don’t cover every situation, so confirm unusual cases before billing.
Why Do COB Problems Start at Registration?
CMS’s MSP booklet states that providers must determine whether Medicare is the primary payer by asking patients at each visit before providing services, checking eligibility for other coverage, and billing the primary payer first. Providers must keep MSP responses for 10 years after the service date.
If a Medicare claim lacks complete other-insurance information, the claims contractor can refer it to CMS’s MSP contractor, which may ask the patient, employer, insurer, or attorney to complete a Secondary Claim Development Questionnaire. If the Medicare record shows another insurer should pay first, Medicare denies the claim. Commercial payers may also send COB questionnaires, and claims can be pending until the member responds. A sound intake process asks about every source of coverage, verifies each plan, and records the payer order and verification date.
How Do You Bill a Secondary Claim?
Bill the primary payer first and post its remittance, including payment, allowed amount, and adjustments, because the secondary claim depends on it. When Medicare is secondary, CMS says it uses the primary payer’s payment information to calculate its payment. Hard-copy claims must include the other insurer’s EOB or remittance advice, and electronic claims must carry the information in the required fields. If the primary is denied or underpaid, the Medicare claim must explain why using the correct claim adjustment reason codes.
Other secondary payers generally don’t pay simply because the primary denied. The reason and plan terms matter. For example, the NAIC model says a secondary plan doesn’t pay an amount the primary reduced because the member skipped precertification. Contractual adjustments, typically group code CO, generally aren’t billable to the patient. Denials for missing COB information are commonly reported with CARC 22.
Quick Reference
| Situation | Primary | Secondary |
| Employee and dependent coverage | Plan covering the patient as employee | Plan covering the patient as dependent |
| Child, parents married or living together | Plan of parent with the earlier birthday | Other parent’s plan |
| Age 65+, employer with 20+ employees | Employer plan | Medicare |
| Disability, employer with 100+ employees | Employer plan | Medicare |
| Medicare and Medicaid | Medicare | Medicaid |
These are general rules. Plan terms, state rules, and exceptions can change the outcome.
Conclusion
Preventing COB errors depends on confirming payer order at registration and posting the primary payer’s remittance accurately. Rapid RCM Solutions supports this through its eligibility and benefits verification, payment posting, and AR and denial management services.
FAQs
What is the birthday rule?
It decides payer order for a child covered under both parents’ plans when the parents are married or living together. The parent whose birthday falls earlier in the calendar year, by month and day, has the primary plan.
When is Medicare the secondary payer?
When a patient 65 or older has coverage through current employment at an employer with 20 or more employees, when a disabled patient has coverage through current employment at an employer with 100 or more, and during the 30-month ESRD period.
Will a secondary payer pay if the primary denies?
Not automatically. Medicare may pay in certain situations, such as an unmet group plan deductible or a service the plan doesn’t cover, if the claim explains the denial. For other payers, it depends on the reason and plan terms.
