Official CARC description: “Exact duplicate claim/service.” Here’s what it actually means, why it fires, and how to get the claim paid.
The payer already has this claim. Real duplicates need no action — but “false duplicates” (legitimate repeat services on the same day) and stuck originals hide underneath this code.
Liability group: CO = Contractual Obligation — the provider absorbs it — the amount cannot be billed to the patient. If the denial is wrong, the money is recovered by correcting or appealing the claim, not by balance-billing.
Also written: CO-18, CO18, “denial code 18”, “18 denial code” or “reason code 18” — same CARC, different ERA formatting.
Reason code 18 also appears under another group on the ERA. The number is the reason; the prefix decides who absorbs the money. This page covers CO 18 (Contractual Obligation). If your ERA shows a different prefix, use the matching page:
Locate the ORIGINAL claim's status first — that decides everything
Legit repeat service: corrected claim with modifier 76/77/91 or RT/LT
Correcting a claim: resubmit with frequency 7 + original claim number
Original mishandled: work THAT claim; another copy just re-denies
Appeal only the false-duplicate case: show the two services were distinct (times, sites, repeat-test rationale) with the right modifier on a corrected claim.
Fill in the denial facts — use placeholders, never real patient data (the letter keeps [PATIENT NAME]-style fields so you can merge real details privately). A payer-ready draft with an enclosure checklist comes back in ~20 seconds.
Your appeal letter draft will appear here.
The official CARC description for CO 18 is: “Exact duplicate claim/service.” In practice: The payer already has this claim. Real duplicates need no action — but “false duplicates” (legitimate repeat services on the same day) and stuck originals hide underneath this code.
Denial code 18 under the CO group (Contractual Obligation) means: Exact duplicate claim/service. The number is the reason; the CO prefix decides who is liable — the provider absorbs it — the amount cannot be billed to the patient. If the denial is wrong, the money is recovered by correcting or appealing the claim, not by balance-billing.
Yes. CO 18, CO-18, CO18 and “denial code 18” are the same CARC — clearinghouses, payer portals and PM systems just print it differently. Only the group prefix (CO, PR, OA or PI) changes the meaning, because it changes who has to absorb the amount.
CO stands for Contractual Obligation — the provider absorbs it — the amount cannot be billed to the patient. If the denial is wrong, the money is recovered by correcting or appealing the claim, not by balance-billing.
Appeal only the false-duplicate case: show the two services were distinct (times, sites, repeat-test rationale) with the right modifier on a corrected claim.
The number 18 is the same reason code in both — what changes is the liability group. OA 18 is Other Adjustment. CO 18 is Contractual Obligation, so the provider absorbs it. Check the prefix on the ERA before you write anything off or bill the patient — a mis-grouped 18 is one of the more winnable appeals.