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

What does code CO-23 mean?

Code 23 means the adjustment reflects "the impact of prior payer(s) adjudication including payments and/or adjustments." You see it on remittances from a secondary payer. It is the line where the secondary says: the primary already paid this much and wrote off that much, so here is the slice of your charge that is no longer in play. X12 assigns code 23 to group code OA, which is why OA-23 is the form you will most often see; some payer systems still print CO-23.

Worked example: you bill $200.00 to Medicare (primary). Medicare allows $100.00, pays $80.00, leaves $20.00 coinsurance. The secondary's ERA shows: billed $200.00, OA/CO-23 adjustment $180.00 (the $80.00 primary payment plus the $100.00 primary contractual write-off), then pays the $20.00 coinsurance. Nothing was denied.

Why does CO-23 appear on the remittance?

Because HIPAA remittance rules require every dollar of the billed charge to be accounted for by reason codes. On a secondary claim, code 23 is the bucket that absorbs everything the primary payer already resolved: its payment plus its contractual adjustment. It appears on essentially every crossover and COB claim. The only time it signals a problem is when the code-23 dollars do not match the primary's EOB, which points to bad COB data on the claim or a keying error in the primary payment fields.

How do you post and reconcile a CO-23 adjustment?

1. Pull the primary ERA for the same claim and confirm: code-23 amount = primary paid + primary contractual adjustment.
2. Post the secondary payment against the remaining patient-responsibility balance, not against the full charge.
3. Do not post code 23 as a second write-off. The primary's contractual was already written off when you posted the primary; double-posting it drives the account negative.
4. If the math does not reconcile, check the COB amounts submitted on the secondary claim (loop 2320/2330 on the 837). Wrong primary-paid figures are the usual culprit; correct and resubmit.

How do you avoid CO-23 posting problems?

Send accurate primary payment data on every secondary claim: primary paid amount, adjustment amounts, and adjudication date. Let crossovers happen automatically where they exist (Medicare to Medigap) instead of billing the secondary manually and racing the crossover. Keep coordination of benefits current at registration so claims flow primary-then-secondary in the right order, and use the denial code lookup when a secondary remit carries code 23 alongside a true denial code; the other code is the one to work.

Can you appeal CO-23?

No, because there is nothing to appeal; code 23 is accounting, not adjudication. What you can dispute is the math behind it. If the secondary calculated its payment from wrong primary figures, resubmit the secondary claim with corrected COB data and the primary EOB attached. If a secondary underpaid coinsurance it owed, that is a payment dispute: send the primary remittance and your calculation, and if the payer stonewalls, escalate through its provider dispute process using the appeal letter generator to document the discrepancy.