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.
Seeing denial code 18 on your EOB?
A denial code tells you what the payer reported, but it may not tell the whole story. Reviewing your EOB and bill together can help clarify what may need attention.
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
- What Is an Itemized Medical Bill? How to Request One
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.
Seeing this code on your own EOB?
Upload the EOB, the bill, or both — MD Bill Check reads the codes on your documents, compares them side by side, and flags what's worth questioning. Free, no account needed.