Denial code · group CO
CO-18: The payer already has this claim or line and treats the new one as an exact duplicate
Short answer
CO-18 means the payer matched this claim or line to one it already received for the same member, provider, service and date, and dropped it as a duplicate. Often the original was paid and the fix is to post it. The rest are a correction sent without frequency code 7, a repeat service billed without its modifier, or a crossover claim billed twice. The 835 for the original claim tells you which.
Look up other codes in the denial code library.
Last reviewed by the Remitwright billing team. Editorial policy
Group CO means contractual obligation: the balance is written off, not billed to the patient. The number is the reason; the two-letter group says who carries the balance.
Remark codes that usually come with CO-18
- M86
- Payment was already made for the same or a similar service within the payer's set time frame
- N111
- No appeal right except on the duplicate question; the service was already billed and adjudicated on an earlier claim
Common causes
The claim was sent twice before the first one finished
A clearinghouse rejection was misread as a payer rejection and the claim was rebilled, or an automatic rebill rule fired while the original was still in process. The payer sees two identical claims and keeps the first.
A corrected claim sent as an original
The fix went out with frequency code 1 instead of 7 and no original claim number. To the payer it is a second claim for the same service, not a replacement, so it denies CO-18 and the original stays exactly as it was.
A repeat service without the modifier that explains it
Two of the same test on one date, a procedure performed twice, or the same code on two lines for left and right. Without 76, 77, 91, RT and LT or whatever the payer's rule calls for, the second line looks identical to the first.
Two providers in the group billed the same thing
A supervising clinician and a rendering clinician both bill the visit, or two locations bill the same patient on the same day. The payer matches on member, date and code and denies whichever arrived second.
A crossover claim billed again
The primary payer forwarded the claim to the secondary automatically, and the office also sent a secondary claim. The secondary denies the one it received second as a duplicate of the crossover.
How to fix it and resubmit
- 1
- 2
Resubmit a correction as a correction
If you meant to replace the original, send it with frequency code 7 and the original claim number from the payer's acknowledgement or 835. For payers that run a separate corrected-claim process, follow it rather than sending another original.
- 3
Add the modifier that distinguishes the repeat
When two services really happened, correct the second line with the modifier the payer expects (76 or 77 for a repeat procedure, 91 for a repeat lab, RT and LT for sides), or combine them into one line with units if that is the payer's rule, and send the correction.
- 4
Reconcile crossover and secondary billing
If the secondary received a crossover from the primary and your own secondary claim, let the crossover stand and write off the duplicate. Check the primary's 835 for the forwarding alert (MA18) before you send any secondary claim.
- 5
Appeal only a mismatch
If the payer's "original" is for a different service or provider and its matching logic is too broad, call with both claim numbers. N111 means the payer will consider only the duplicate question, so the appeal needs to show the two services were different, not that the service was necessary.
How to prevent it
- Claim status worked from the 277CA and payer acknowledgements before any claim is rebilled, with no automatic rebill inside the payer's processing window
- Corrected claims always sent with frequency code 7 and the original claim number, through the payer's stated corrected-claim route
- Modifier logic in charge entry for repeat procedures, repeat labs and bilateral sides, matched to each payer's rule
- A crossover indicator on Medicare patients so the secondary claim is held until the primary 835 shows whether it was forwarded
A CO-18 on two NEMT trips in one day
An NEMT provider takes a member to dialysis in the morning and home in the afternoon and bills two lines with the same transport code, the same date and the same units, with no modifiers. The Medicaid plan pays the first line and denies the second CO-18. The plan's transportation guide requires origin and destination modifiers on each leg. The biller adds the modifiers that identify the home-to-facility and facility-to-home legs, resubmits with frequency code 7, and both legs pay. The dispatch export is changed so the modifiers come through automatically. This is a hypothetical example.
Official text: the X12 claim adjustment reason code list. The meaning above is our paraphrase for billers; the list is the authority.
FAQ
Questions about CO-18
Why do I see CO-18 instead of OA-18?
The original claim was denied. Why is my resubmission a duplicate?
Can I appeal a CO-18?
How do I bill two of the same service on one day?
Is CO-18 a timely filing risk?
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