Billing a claim secondary is not the same as billing it primary with the EOB attached. Coordination of Benefits (COB) claims have their own timely filing windows, documentation requirements, and adjudication logic — and each major payer handles them slightly differently. Getting this wrong means denials on claims that would otherwise be paid.
The COB Timely Filing Clock
This is the most commonly misunderstood element of COB billing. For secondary claims, timely filing does not run from the date of service. It runs from the date of the primary payer's EOB. Here's how the major payers handle it:
- UnitedHealthcare: 90 days from the primary EOB date for commercial plans; 12 months for MA
- Aetna: 180 days from the primary EOB date for commercial; 12 months for MA
- BCBS: Varies by affiliate — typically 90–180 days from primary EOB; MA plans 12 months
- Cigna: 90 days from primary EOB for most commercial products
- Humana: 90 days from primary EOB for commercial; 12 months for MA
- Medicaid (as secondary): Generally 12 months from DOS or primary EOB date; varies by state
The key takeaway: your billing system needs to store the primary EOB date and calculate the secondary filing deadline from that date — not from DOS.
What to Attach to the Secondary Claim
Every secondary claim requires the primary payer's EOB as documentation. For electronic submissions (837P/837I), the primary adjudication information goes in the COB data segment — Loop 2320 and its related loops (2330A, 2330B, etc.). For paper claims, attach the primary EOB to the CMS-1500 or UB-04.
The EOB must show: primary payer name, payment amount (or $0 with denial reason), patient responsibility amount, and the claim-level adjustment reason codes (CO, PR, OA). Without this information, the secondary payer cannot process COB correctly.
Crossover Claims vs. Manual COB
For Medicare/Medicaid dual eligibles, many claims cross over automatically — Medicare transmits the claim directly to the state Medicaid agency after processing. If the crossover doesn't happen (you'll see this when Medicaid has no record of the claim), you'll need to file manually and attach the Medicare EOB. Check your state Medicaid's timely filing rules for manual COB submissions — they often differ from standard Medicaid filing windows.
Non-Duplication vs. Coordination of Benefits
Some secondary plans use a non-duplication COB method rather than standard COB. Under non-duplication, the secondary pays nothing if the primary paid at least as much as the secondary would have paid on its own. Under standard COB, the secondary pays up to the patient's actual cost-sharing after primary adjudication. Know which method applies — it affects whether you should even bother submitting secondary and how you set patient balance expectations.
Medicare as Secondary (MSP)
Medicare Secondary Payer rules are federal, and the penalties for non-compliance are significant. When another payer has primary responsibility (employer group health, liability, no-fault, workers' comp), Medicare must be billed secondary. Submit to the primary first, wait for the EOB, then submit to Medicare with the MSP Claim Input Form data or proper 837 COB loops. The MSP timely filing window is 12 months from the primary EOB date.
Bottom Line
COB billing is a discipline unto itself. The timely filing clock runs from the primary EOB, not DOS. Store that date in your system. Know your payers' non-duplication vs. coordination methods. And for Medicare Secondary Payer situations, treat MSP compliance as a hard requirement — the exposure on non-compliance exceeds the value of the claim.