Denial Code 18 (CO-18, PR-18) — Meaning & What to Do
Claim Adjustment Reason Code (CARC)
What denial code 18 means: This looks like a duplicate of a claim already submitted.
What to do about denial code 18: If it truly is a duplicate, no payment is owed. Ask the provider to confirm it isn't billed twice.
CO-18 vs PR-18 vs OA-18: what the prefix means
| Code on your EOB | What the prefix means | Who typically pays |
|---|---|---|
| CO-18 | Contractual obligation — the provider writes the amount off | Usually not the patient |
| PR-18 | Patient responsibility | The patient |
| OA-18 | Other adjustment | Often another payer or plan |
The prefix tells you who is expected to absorb the amount; the number (18) tells you why.
Related codes
CARC 16The claim is missing information or has an error that must be fixed before it can be processed.
CARC 22Another insurance should be billed first (you may have primary coverage elsewhere).
CARC 4The procedure code doesn't match a required modifier, or the modifier is missing/invalid.
CARC 23Payment was affected by a prior payer's decision (coordination of benefits).
Keep reviewing your bill
- Medical Bill vs EOB: What Is the Difference?
- How to Dispute a Medical Bill — Find Billing Errors Before You Pay
Frequently asked questions about Denial code 18
What does denial code 18 mean on my EOB?
This looks like a duplicate of a claim already submitted.
What should I do about denial code 18?
If it truly is a duplicate, no payment is owed. Ask the provider to confirm it isn't billed twice.
Found this code on your bill or EOB?
Upload the document — MD Bill Check decodes every code, checks the math line by line, and explains what's worth questioning.