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CO-189 Denial Code: Unlisted Code Used, Specific Exists

CO-189 means the claim used a 'not otherwise classified' (NOC) or unlisted procedure code when a specific CPT or HCPCS code exists for the service. The payer wants the specific code, so the usual fix is a corrected claim.

Quick facts

Code
CO-189 (CARC 189)
Status
Active In use since June 30, 2005.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The provider is responsible for the coding error. The amount is not billable to the patient.
Official description
'Not otherwise classified' or 'unlisted' procedure code (CPT/HCPCS) was billed when there is a specific procedure code for this procedure/service
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-189 means

CARC 189 reads “‘Not otherwise classified’ or ‘unlisted’ procedure code (CPT/HCPCS) was billed when there is a specific procedure code for this procedure/service.” Unlisted codes (typically ending in 99 in CPT sections) and NOC HCPCS codes are catch-alls for services with no specific code. Payers price them manually, so they require more work and are scrutinized closely. When a specific code exists, payers expect you to use it.

Example: a practice bills a newer procedure with an unlisted code out of habit. A Category III code was created for that procedure, so the payer denies the unlisted line with CO-189. The practice rebills with the specific code.

A similar pattern appears with drugs: a NOC drug HCPCS code is billed after the drug received its own specific HCPCS code in a quarterly update.

Common causes

  • New specific codes released in an annual CPT or quarterly HCPCS update that replace a formerly unlisted service.
  • Habitual use of unlisted codes in templates or charge masters.
  • Drugs billed with NOC codes after a product-specific HCPCS code became effective.
  • Category III codes overlooked, though CPT guidance says to use them before unlisted codes.
  • Coder uncertainty about which specific code applies.

How to fix it

  1. Search current CPT and HCPCS code sets for a specific code that describes the service on the date of service.
  2. Check the payer’s crosswalk or drug billing guidance for recently coded products.
  3. Submit a corrected claim with the specific code (resubmission code 7 in box 22 with the original claim number).
  4. If no specific code exists, appeal with the operative report or documentation, a description of the service, and a comparison code to help the payer price it.
  5. Update templates so the unlisted code isn’t used again.

How to prevent it

  • Review unlisted and NOC code usage after every code set update.
  • Crosswalk NOC drug codes to product-specific codes as they’re released.
  • Train coders to check Category III codes before unlisted codes.
  • Flag unlisted codes for coder review before submission. The Claims Validator can catch codes that need a second look before the claim goes out.

Specialty notes

Surgical specialties and infusion or oncology practices see this code most often, because both new procedures and new drugs frequently move from unlisted or NOC codes to specific codes.

Remark codes that may appear with CO-189

  • N56 (Procedure code billed is not correct/valid for the services billed or the date of service billed.): The procedure code billed isn't correct or valid for the service described.
  • N350 (Missing/incomplete/invalid description of service for a Not Otherwise Classified (NOC) code or for an Unlisted/By Report procedure.): The description for the NOC or unlisted code is missing or invalid.
  • M51 (Missing/incomplete/invalid procedure code(s).): The procedure code is missing, incomplete, or invalid.
  • CO-181 (Procedure code was invalid on the date of service.): The code billed wasn't valid on the date of service.
  • CO-16 (Claim/service lacks information or has submission/billing error(s).): Missing or invalid claim information, such as a missing NOC description.
  • CO-4 (The procedure code is inconsistent with the modifier used.): The modifier doesn't fit the procedure code, a different coding problem.

CO-189 FAQ

When is it appropriate to bill an unlisted code?

Only when no specific CPT or HCPCS code describes the service. CPT guidance directs coders to use the specific code (including Category III codes where applicable) before choosing an unlisted code.

What does a payer need with an unlisted code?

Usually a clear description of the service in the claim's note field or box 19, and often supporting documentation such as an operative report, invoice, or comparison to a similar code.

Should I appeal CO-189?

Only if no specific code actually exists for the service performed. Otherwise, correct the code and resubmit.

Why do payers dislike unlisted codes?

They can't be priced automatically. Each one needs manual review and pricing, often against a comparable code, which slows payment and invites more scrutiny than a specific code.