CO-18 at a glance
CARC 18 | |
|---|---|
CARC 18 text (X12) | Exact duplicate claim/service (Use only with Group Code OA except where state workers' compensation regulations requires CO) |
Usual group code | OA, Other Adjustment. CO only under state workers' compensation rules |
In the X12 list since | January 1, 1995. Last modified June 2, 2013 |
What it means for a therapy practice | The payer matched this line to one it already has for the same client, date, code and charge |
Who owes the line | Nobody, on this claim. Payment, if any, comes on the original |
Usual action | Find the original claim and check its status before sending anything else |
Why CARC 18 is meant for OA, not CO
The group code says who carries the adjustment. CO puts it on the provider under the contract, and PR puts it on the patient. A duplicate fits neither: the money isn't lost, it's just being handled on another claim. That's why X12 limits CARC 18 to OA, the group for adjustments that aren't the provider's or the patient's responsibility.
The one exception in the code text is state workers' compensation, where some state regulations require CO. So if a CO‑18 arrives on a workers' compensation claim, it's probably the payer following its state rule. If it arrives from a commercial plan or Medicaid, treat the duplicate finding the same way, but don't assume the CO group is correct without asking.
How duplicate denials happen in therapy billing
Scenario | What went wrong | Fix |
|---|---|---|
No payment after a few weeks, so the claim was sent again as a new claim | The first claim was still in process | Check the claim status first. If the original was received, wait or follow up on that one |
A billing error was fixed and the claim sent again as an original | A correction needs to go as a replacement of the earlier claim, not a fresh one | Resubmit as a corrected claim that references the original claim number |
The same claim went out through the clearinghouse and the payer portal | Two channels, one service | Pick one submission route per payer |
Two separate sessions for one client on one day, same CPT code | Identical lines look like a duplicate even when they aren't | Ask the payer how it wants a second distinct session reported, and document both start and stop times |
A secondary claim was sent to the primary payer by mistake | The primary already processed it | Send the secondary claim, with the primary's remittance, to the secondary payer |
The second row is the easiest to miss. Billing software often defaults every claim to an original. If you change the diagnosis or a modifier and send it again without marking it as a replacement, the payer sees two claims for one session and denies the newer one.
Duplicate denial code vs similar codes
Code | X12 text | How it differs from CARC 18 |
|---|---|---|
18 | Exact duplicate claim/service (Use only with Group Code OA except where state workers' compensation regulations requires CO) | The same line was already received |
97 | The benefit for this service is included in the payment/allowance for another service/procedure that has already been adjudicated. | A different service was paid, and this one is bundled into it |
23 | The impact of prior payer(s) adjudication including payments and/or adjustments. (Use only with Group Code OA) | The adjustment reflects what an earlier payer did, not a duplicate |
129 | Prior processing information appears incorrect. At least one Remark Code must be provided (may be comprised of either the NCPDP Reject Reason Code, or Remittance Advice Remark Code that is not an ALERT.) | The payer disputes information about earlier processing, often on a corrected or secondary claim |
How to work a CO-18 or OA-18
- Search your billing system and the payer portal for another claim with the same client, date of service and CPT code.
- If the original was paid, post the duplicate line as a zero adjustment and close it. Don't bill the client.
- If the original is still pending, follow that claim instead of sending a third copy.
- If the original was denied for another reason, fix that reason and send a corrected claim that replaces the original.
- If there truly were two sessions, call the payer, ask how it wants them reported, and send the documentation for both.
Example, fictional practice. A solo therapist bills 90837 for a client's session on September 8. By October 10 nothing has come back, so she sends the same claim again. On October 14 the first claim pays, and on October 21 the second comes back OA‑18. Nothing to fix: she posts the duplicate line to zero and makes a note to check claim status before resending next time.
Session codes these claims usually carry: 90837 and 90832 for individual therapy, 90847 for family sessions and 90791 for the intake.
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Related denial codes: CO-129 for incorrect prior processing information, CO-252 when the payer needs an attachment, CO-24 for capitated care and CO-45 for an ordinary fee schedule write‑off.
Checked against the X12 Claim Adjustment Reason Code list in November 2026. CARC 18 was last modified on June 2, 2013, and nothing about it changed this year.