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CO-B18 Denial Code (Deactivated): Code/Modifier Invalid

CO-B18 meant the procedure code and modifier billed were not valid on the date of service. X12 deactivated it without naming a replacement; CARC 181 now covers an invalid procedure code and CARC 182 an invalid modifier.

X12 deactivated CARCB18 on March 1, 2009. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Code: CO-B18 (CARC B18)
Status: Deactivated StoppedMarch 1, 2009 (in use since January 1, 1995).
Code set: Claim Adjustment Reason Codes (CARC)
Group codes: CO (Contractual Obligation): Coding validity is the provider's responsibility, so the amount was not billed to the patient.
Official description: This procedure code and modifier were invalid on the date of service.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What CO-B18 meant

CARC B18 combined two validity problems. Either the procedure code, or the modifier attached to it, was not valid for the date the service was performed. This often happened around annual code set updates when a practice’s charge master had not been refreshed.

What replaced it

X12 did not name a replacement, but the two halves now have separate active codes:

CO-181 when the procedure code was invalid on the date of service.
CO-182 when the modifier was invalid on the date of service.

If the code and modifier are both valid but do not belong together, payers use CO-4.

If you still see CO-B18

It is limited to older denials. When you meet a validity denial today, check the effective and termination dates of the code and modifier against the date of service, and correct your charge master or fee schedule if an outdated code is still active in your system. Resubmit a corrected claim with the valid code.

CO-181 (Procedure code was invalid on the date of service.): Procedure code was invalid on the date of service.
CO-182 (Procedure modifier was invalid on the date of service.): Procedure modifier was invalid on the date of service.
CO-4 (The procedure code is inconsistent with the modifier used.): The procedure code is inconsistent with the modifier used.

CO-B18 FAQ

How does a code become invalid on a date of service?

Procedure codes and modifiers are added, revised, and deleted on set update cycles. Billing a deleted code after its end date, or a new code before its effective date, makes it invalid for that service date.

What is the fix?

Look up the code set in effect on the date of service, replace the invalid code or modifier with a valid one supported by the documentation, and submit a corrected claim.

Important: read before using this information

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