CO-23 Denial Code: Impact of Prior Payer Adjudication (Secondary Claims)

What Does CO-23 Mean?

Code 23 — usually seen as OA-23 — is the least understood code on secondary remittances, because it isn't a denial at all. It reports the impact of the primary payer's adjudication: the amount the primary already paid or adjusted, which the secondary is subtracting before calculating its own payment. Seeing 23 on a secondary EOB is normal arithmetic, not a problem.

Code group: OA/CO (informational adjustment) · Appealable: Only when the prior-payer amounts used were wrong. Otherwise it's normal secondary-claim math — post it correctly.

Why CO-23 Happens

You billed a secondary payer after the primary paid, and the secondary is showing the primary's payment and contractual adjustments as an offset — exactly as coordination of benefits is supposed to work.
The secondary paid less than expected, and the 23 amount explains why: primary payment plus adjustments consumed most of the allowed amount.
Medigap and crossover claims display 23 routinely as the Medicare payment flows through.
The one real problem case: the 23 amount doesn't match what the primary actually paid — a data mismatch between the claim you submitted and the primary EOB.

How to Fix a CO-23 Denial — Step by Step

First: confirm nothing is actually wrong. Compare the 23 amount against the primary's EOB. If they match, the remittance is simply showing correct COB math — post it and move on.

If the secondary paid $0, check whether that's correct: when the primary's payment meets or exceeds the secondary's allowed amount, zero is the right answer, not a denial to fight.

If the 23 amount doesn't match the primary EOB, the secondary adjudicated against wrong prior-payer data — resubmit with the correct primary EOB attached or correct the COB fields on the claim.

Verify the remaining patient responsibility: after both payers, PR amounts on the secondary remittance are what the patient owes — often nothing.

Train posters on code 23 specifically; mis-posting it as a denial creates phantom AR and wasted follow-up calls.

What To Do With CO-23 (Hint: Usually Nothing)

Adapt this wording to your claim — replace the bracketed fields and attach the documentation it references:

There's nothing to appeal in a correct CO-23/OA-23 — it's arithmetic. When the numbers are wrong: 'Secondary claim [number] was adjudicated using prior-payer payment data of [$X]; the primary payer's remittance (attached) shows actual payment of [$Y] and contractual adjustment of [$Z]. We request reprocessing with the corrected prior-payer amounts.'

How to Prevent CO-23

Submit secondary claims with complete, accurate primary EOB data — mismatches cause the only real 23 problems.
Teach payment posters what 23 means so it never gets worked as a denial.
Reconcile secondary payments against the COB math for your top payer pairs monthly.

Related Denial Codes

OA-23 — same code, official group prefix CO-22 — COB / other payer PR-1 — Deductible PR-2 — Coinsurance

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Reviewed by the LegitMedBilling denial management team. This guide is general billing information, not legal or payer-specific advice — always verify against your payer contract and current policy.

Frequently Asked Questions

Is CO-23 a denial?

No — and that's the whole point. It reports how much the primary payer's payment and adjustments reduced what the secondary considers. It's coordination-of-benefits arithmetic on the remittance. The only time it needs action is when the amounts don't match the primary's actual EOB.

The secondary paid $0 and showed code 23. Did we lose money?

Usually not. If the primary's payment met or exceeded the secondary's allowed amount for the service, the secondary correctly owes nothing — the code 23 line shows the math. Compare allowed amounts before assuming a problem; zero-pay secondaries are often correct.

Why does our AR show a pile of 'CO-23 denials' to work?

Because someone posted informational adjustments as denials — one of the most common posting errors in billing. Those aren't workable claims; they're arithmetic lines. Clean them up, then fix the posting rule so code 23 posts as an adjustment reference, not a denial queue item.