A coordination of benefits (COB) denial usually means your insurer thinks another plan might pay first, or it’s waiting for you to confirm whether you have other coverage. Call the member services number on your card, or fill in the plan’s COB questionnaire, and say what other coverage you have (or that you have none) and the dates. Then ask the plan to reprocess your denied claims, and ask your providers to rebill. If Medicare is involved, the Benefits Coordination & Recovery Center (BCRC) keeps Medicare’s records.
Coordination of benefits is the set of rules that decides which plan pays first (primary) and which pays second (secondary) when you have more than one. Many insurers ask members to confirm other coverage from time to time, and claims can stall until they get an answer.
Key takeaways
- On the billing side, the denial usually comes back as CO-22: “This care may be covered by another payer per coordination of benefits.”
- If you have no other coverage, say so. A quick update often clears every frozen claim at once.
- With Medicare and job-based coverage, the employer’s size matters: 20 or more employees if you’re 65 or older, 100 or more if you have Medicare because of a disability.
- For people with Medicare because of end-stage renal disease (ESRD), the group plan pays first for a 30-month coordination period.
How this guide was checked: Medicare rules come from Medicare.gov and CMS, the state rule example comes from South Carolina’s COB regulation, and code wording comes from X12. Last reviewed 4 October 2026. This is educational content, not legal advice.
Table of Contents
Why your claim was denied for COB
From the billing side, COB denials come in a few flavors:
| Code on the remittance | Official X12 wording | What it usually means |
|---|---|---|
| CO-22 | “This care may be covered by another payer per coordination of benefits.” | The plan thinks another plan is primary, or needs you to confirm |
| CO-109 | “Claim/service not covered by this payer/contractor. You must send the claim/service to the correct payer/contractor.” | Billed to the wrong plan |
| N479 (remark) | “Missing Explanation of Benefits (Coordination of Benefits or Medicare Secondary Payer).” | The secondary plan needs the primary plan’s EOB |
| OA-23 | “The impact of prior payer(s) adjudication including payments and/or adjustments.” | Normal secondary processing, not a denial |
Our CO-22 denial code page explains how billers work these, and the full denial code list covers the rest.
Common triggers include an old plan still on file after you changed jobs, a spouse’s plan the insurer found, a child covered by both parents, a new Medicare enrollment or an unanswered COB questionnaire.
Who pays first? Common situations
Medicare and other coverage
| Your situation | Who pays first |
|---|---|
| 65 or older, covered by a current employer plan (yours or your spouse’s), employer has 20 or more employees | The employer plan |
| 65 or older, current employer plan, employer has fewer than 20 employees | Medicare |
| Under 65 with Medicare because of a disability, current employer plan with 100 or more employees | The employer plan |
| Under 65 with a disability, employer has fewer than 100 employees | Medicare |
| Medicare because of ESRD, with a group plan or COBRA | The group plan or COBRA, for a 30-month coordination period. Then Medicare |
| 65 or older, retired, with retiree coverage | Medicare |
| Medicare (because of age or disability) and COBRA | Medicare |
Sources: Medicare.gov “Who pays first?” and “How Medicare works with other insurance,” and CMS’s Medicare Secondary Payer and ESRD pages. Multi-employer plans have their own size rules, so check with your benefits office.
If the primary insurer doesn’t pay promptly, which Medicare.gov describes as usually within 120 days, Medicare may make a “conditional payment” and recover it later. If the wrong plan paid first and the provider now has to refund it, our biller guide on tertiary insurance overpayments explains the provider side. Our biller guide on billing Medicare as secondary shows how the secondary claim works.
Two private plans (state rules)
For two private group plans, states set the order with COB rules. South Carolina’s regulation 69-43 is one example. Its order:
- Employee before dependent. The plan that covers you as the employee or subscriber pays before the plan that covers you as a dependent.
- Children: the birthday rule. For a child whose parents aren’t separated or divorced, the plan of the parent whose birthday comes earlier in the calendar year pays first. Only the month and day count, not the year.
- Children of separated or divorced parents. A court decree that makes one parent responsible for health care costs controls, once the plan knows about it. Without one, the custodial parent’s plan pays first, then the custodial parent’s spouse’s plan, then the other parent’s plan.
- Active before laid off or retired. A plan covering you as an active employee pays before one covering you as a laid-off or retired employee.
- Longer coverage first. If nothing else decides it, the plan that has covered you longer pays first.
Your state’s rules may differ, so check your plan documents or your state insurance department.
Step-by-step: how to fix a COB denial
- Read the EOB or letter. Look for “other insurance,” “coordination of benefits” or code CO-22.
- Call member services. Ask: “What other coverage do you have on file for me, and what do you need to update it?” Note the reference number.
- Update your COB information. Use the plan’s form, online portal or phone line. Give the other plan’s name, member ID and start and end dates, or confirm you have no other coverage.
- For Medicare records, call the BCRC at 1-855-798-2627 (TTY 1-855-797-2627), which Medicare.gov lists for updating other coverage.
- Ask the plan to reprocess the denied claims. Some plans do this automatically, and some need each provider to rebill.
- Tell your providers. Ask them to update your insurance order and rebill the right plan first. Send them the secondary plan’s details too.
- Watch the EOBs. Each reprocessed claim should come with a new EOB. Compare them using our guide on how to read an Explanation of Benefits.
If a COB denial dragged on and the provider missed a filing deadline, see our guide to medical bills that arrive months later. If a balance already went to collections, see how to dispute a medical bill in collections.
Free template: COB update letter
[Your name] | [Address] | [Phone]
Member ID: [ ] Group #: [ ]
[Date]
[Insurer] - Coordination of Benefits / Member Services
[Address or fax from the insurer's website]
Re: Coordination of benefits update - please reprocess claims
Please update your coordination of benefits records for:
Member(s): [names and dates of birth]
Choose one:
[ ] I have NO other health coverage as of [date].
[ ] I have other coverage:
Insurer: [ ] Policyholder: [name, date of birth]
Member ID: [ ] Group #: [ ]
Coverage start date: [ ] End date (if any): [ ]
Coverage type: [employer / spouse's employer / Medicare /
Medicaid / COBRA / retiree / individual]
[ ] My other coverage ENDED on [date]: [insurer, member ID]
Please reprocess all claims denied for coordination of benefits
since [date], including claim #[ ], and send me updated
Explanations of Benefits. Please confirm in writing when the
update is complete.
Sincerely,
[Signature]Phone script
“Hi, I’m calling to update coordination of benefits for member ID [number]. I [have no other coverage / have other coverage with (insurer) starting (date)]. Can you update the record today, reprocess the claims denied for COB since (date), and give me a reference number?”
FAQ
Why was my claim denied for coordination of benefits if I only have one insurance?
The plan may have old coverage on file or may be waiting for you to confirm you have no other insurance. Call member services, confirm you have no other coverage, and ask them to reprocess the denied claims.
What does CO-22 mean on my EOB?
CO-22 is the standard code for “This care may be covered by another payer per coordination of benefits.” It means the plan won’t pay until it knows whether another plan should pay first.
Which plan pays first for my child if both parents have insurance?
Under the common birthday rule, used for example in South Carolina’s regulation, the plan of the parent whose birthday comes first in the calendar year pays first. Only month and day count. Different rules apply for separated or divorced parents.
Does Medicare or my employer plan pay first?
If you’re 65 or older and covered through a current employer with 20 or more employees, the employer plan usually pays first. With fewer than 20 employees, Medicare usually pays first. For disability-based Medicare, the cutoff is 100 employees.
How do I update my coordination of benefits with Medicare?
Contact the Benefits Coordination & Recovery Center at 1-855-798-2627 (TTY 1-855-797-2627), as Medicare.gov advises, and tell your providers about any changes.
Related patient guides
Disclaimer: This article is educational and is not legal advice. Coordination of benefits depends on your plans’ terms, state rules and federal Medicare Secondary Payer rules. Contact your plans and benefits office about your coverage, and check each linked source, since rules can change.
Written by Manikandan, RCM specialist at Medical Billing 101.
Sources
- Medicare.gov, Who pays first?, retrieved 4 Oct 2026.
- Medicare.gov, How Medicare Works with Other Insurance (publication 02179), retrieved 4 Oct 2026.
- CMS, Medicare Secondary Payer and End-Stage Renal Disease (ESRD), retrieved 4 Oct 2026.
- S.C. Code Regs. 69-43, Group Health Insurance Coordination of Benefits (via Cornell LII), retrieved 4 Oct 2026.
- X12, Claim Adjustment Reason Codes and Remittance Advice Remark Codes, retrieved 4 Oct 2026.
Manikandan is a Revenue Cycle Management (RCM) specialist with over 10 years of hands-on experience in US healthcare billing. He has worked extensively with commercial payers, Medicare, and Medicaid across multiple specialties including surgery, orthopedics, and radiology. Manikandan founded Medical Billing 101 to provide free, accurate denial code guides, CPT coding references, and Medicare billing resources for US medical billing professionals.
