CO-23: impact of prior payer adjudication.

What X12 says CARC 23 means

The impact of prior payer(s) adjudication including payments and/or adjustments. (Use only with Group Code OA)

CARC 23 · Start 01/01/1995, last modified 09/30/2012.

In plain terms: A report of what an earlier payer paid or adjusted, so this payer's amount reflects it. X12 says to use it only with group code OA.

What it means on a remittance

By its wording, CARC 23 explains an amount: this payer's figure was changed by the payments and adjustments of one or more payers before it. Read that way, it describes an amount and does not by itself say the service is not covered. The X12 text ends with “(Use only with Group Code OA)”, so CO-23 combines a code with a group the usage note excludes. Software, clearinghouses and portals sometimes show the group differently from the raw 835, so look at the raw remittance before deciding.

Typical causes

• A secondary payer reduced its payment because the primary already paid or adjusted the line.
• The primary payer's figures on the claim did not match what the primary reported on its own remittance.
• The payer or your system displayed the wrong group code for an OA adjustment.

These are common billing patterns, not a diagnosis of your claim. The remittance and the payer's policy decide.

What to check

1. Lay the primary payer's remittance beside this one and add up: charge, primary paid, primary adjustments, this payer's paid and adjustments.
2. Check that the claim sent the primary's paid amount and adjustment amounts exactly as reported.
3. Look at the raw 835 for the group code on this adjustment, not only the portal view.
4. Ask the payer to confirm the group code if the amounts balance but the group is CO.

Correct, resubmit or appeal

If the amounts balance, there is usually nothing to appeal; the line is the arithmetic of a secondary claim. If they do not, correct the primary payer figures on the claim and resubmit. If you disagree with the payment, request review through the payer's process. For Medicare the window is 120 days from receipt of the notice, with receipt presumed 5 days after the notice date.

Medicare dates that apply

Medicare claims must be filed no later than the close of the period ending 1 calendar year after the date of service (42 CFR 424.44(a)(1)), with exceptions in 424.44(b). A redetermination request must be filed within 120 calendar days from the date a party receives the notice, receipt is presumed 5 days after the notice date (42 CFR 405.942(a)), and a late request can be considered for good cause (405.942(b)). Clerical errors, including inaccurate data entry, are handled by the contractor as reopenings instead of redeterminations (42 CFR 405.980(a)(3)). The tool above runs the date math.

Related codes

X12 does not publish a table that pairs each CARC with remark codes; the payer picks the remark code and prints it beside the CARC on the same remittance line. These X12 remark codes carry wording that speaks to this reason, so look for them:

• N4 — Missing/Incomplete/Invalid prior Insurance Carrier(s) EOB.
• N479 — Missing Explanation of Benefits (Coordination of Benefits or Medicare Secondary Payer).
• N480 — Incomplete/invalid Explanation of Benefits (Coordination of Benefits or Medicare Secondary Payer).

Other codes: CARC 45 (charge over fee schedule), CARC 109 (claim sent to the wrong payer). The full lookup reads any code on this page.

Worked example (hypothetical)

A $200 line: the primary pays $120 and writes off $50. The secondary's remittance shows CO-23 for the $70 impact of the primary and pays nothing further. The biller confirms the primary figures on the claim match the primary's remittance and closes the line.

Sources

• X12: Claim Adjustment Reason Codes — code text, usage notes and dates (list dated 10/30/2026, retrieved 2026-11-02)
• X12: Remittance Advice Remark Codes — remark code text (retrieved 2026-11-02)
• 42 CFR 424.44, 42 CFR 405.942 and 42 CFR 405.980 — eCFR, current through 2026-10-29 (retrieved 2026-11-02)

Code descriptions are X12 copyrighted material, quoted here only for the codes on this page and linked to the owner. For the full lists, use X12. Every rule above was read from these pages on November 2, 2026. Payers revise code lists, contracts and policies, so confirm anything that affects a claim or a deadline with the payer.