CO-23CO group · Coordination of benefits

CO-23 denial code: adjusted for the prior payer's adjudication

CO-23 is arithmetic, not judgement — which means when it is wrong, it is wrong for a reason you can point at.

CO-23 appears on secondary claims and reflects the impact of the primary payer's adjudication on what the secondary owes. It is frequently correct. When it is not, the cause is almost always a misread of the primary remittance rather than a coverage dispute.

What CO-23 means

A secondary payer calculates its liability from the primary's allowed amount, payment and patient responsibility. Errors enter when the primary's contractual adjustment is treated as patient responsibility, when a coordination method is applied that the plan document does not use, or when the secondary calculates against its own fee schedule where the plan calls for a different method. Each of those is checkable against the two remittances side by side.

Why CO-23 fires

  • The primary paid and the secondary is coordinating its share correctly.
  • The primary's contractual adjustment was read as patient responsibility.
  • The secondary applied a coordination method other than the one the plan specifies.
  • Amounts were transposed when the primary remittance was keyed onto the secondary claim.

Is CO-23 worth appealing?

Sometimes worth appealing

Reconcile the two remittances line by line before disputing. Where the coordination method is the issue, the plan document decides it and quoting it is the appeal.

How to resolve or appeal CO-23

  1. 1

    Lay the remittances side by side

    Allowed, paid, contractual adjustment and patient responsibility, per line.

  2. 2

    Name the coordination method

    Where the calculation implies a method the plan does not use, quote the plan language.

CO-23 — frequently asked

Is CO-23 a denial?
Usually an adjustment rather than a denial — it explains why the secondary paid less, not that it refused. Treating it as a denial leads to appeals that miss the point.
Why does the arithmetic go wrong so often?
Because the primary's contractual adjustment and the patient's responsibility look similar on a remittance and behave completely differently in the secondary's calculation.

Related guides

OA-23 · Coordination of benefitsCO-129 · AdministrativeCO-22 · Coordination of benefits

Sources

Reason-code meanings are paraphrased from the X12 Claim Adjustment Reason Code list for plain-language reference; they are not reproduced verbatim. This is general information, not legal, coding, or medical advice — always confirm against the payer's remittance and policy.

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