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CO-4 Denial Code: Procedure Inconsistent With Modifier

CO-4 means the payer found the procedure code inconsistent with the modifier you used: the modifier is missing, invalid for that code, or conflicts with payer policy. Correct the modifier and send a corrected claim; the provider is responsible under CO.

Quick facts

Code
CO-4 (CARC 4)
Status
Active In use since January 1, 1995; last modified March 1, 2020.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The usual group. The denied amount is the provider's responsibility and cannot be billed to the patient; fix the modifier and resubmit.
  • PR (Patient Responsibility): Less common. The payer assigns the amount to the patient, for example where the patient accepted financial responsibility. Check the remark codes and any signed waiver before billing.
Official description
The procedure code is inconsistent with the modifier used. Usage: Refer to the 835 Healthcare Policy Identification Segment (loop 2110 Service Payment Information REF), if present.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-4 means

CARC 4 says the procedure code is inconsistent with the modifier used. Payers check each modifier in box 24D against the procedure code it is attached to. If the modifier is not valid for that code, is required but missing, or contradicts the payer’s policy for the service, the line is denied with CO-4.

The official usage note points to the 835 Healthcare Policy Identification segment (loop 2110 REF). When the payer populates it, that policy reference tells you which rule the line failed, which is often faster than calling.

Example: a practice bills a code with modifier 26 (professional component), but the code has no professional or technical split. The payer cannot apply modifier 26 to it, so the line returns CO-4 with a remark such as N823.

Common causes

  • Component modifiers on the wrong codes. Modifiers 26 and TC used on procedures that are not split into professional and technical components.
  • Missing required modifiers. Therapy modifiers (GP, GO, GN) on PT/OT/speech services, telehealth modifiers (95, GT, or 93 for audio-only, depending on payer), or DME modifiers (NU, RR, UE).
  • Laterality errors. RT, LT, or 50 on a code that is already bilateral, or anatomic modifiers on codes where they do not apply.
  • Modifier 25 or 57 on a non-E/M code. These modifiers are meant for evaluation and management services.
  • Invalid combinations. Two modifiers that contradict each other on one line (see N519).
  • Payer-specific modifier rules. Some commercial and Medicaid plans require their own modifiers, for example for provider credential levels in behavioral health.

How to fix it

  1. Read the remark code and any policy reference on the ERA to identify the problem modifier.
  2. Check the code’s modifier rules in the payer’s policy and, for Medicare, the physician fee schedule indicators (for example PC/TC and bilateral indicators).
  3. Review the documentation to confirm what was actually performed and which modifier, if any, it supports.
  4. Send a corrected claim. Fix box 24D, enter resubmission code 7 and the original claim reference number in box 22, and resubmit. This is a correction, not an appeal.
  5. Appeal only when you were right. If the modifier was valid under the payer’s published policy, request reconsideration with the policy citation and documentation.
  6. Do not bill the patient. CO means the provider is responsible for the denied amount.

How to prevent it

  • Maintain a modifier matrix by payer for your most-billed codes, including required therapy, telehealth, and component modifiers.
  • Update it when fee schedule indicators and payer policies change, typically each January and quarterly.
  • Validate modifiers before submission. A Claims Validator check can flag a modifier that does not fit the procedure before the claim goes out.
  • Train coders on modifier intent, especially 25, 59 and the X modifiers, where misuse also causes bundling denials. See NCCI denials and modifiers.

Specialty notes

PT/OT/speech practices see CO-4 most often from missing or mismatched GP/GO/GN modifiers. DME suppliers see it when rental and purchase modifiers conflict with the item billed. Radiology and cardiology groups see it with 26/TC on global-only codes.

Remark codes that may appear with CO-4

  • N822 (Missing procedure modifier(s).): A required modifier was missing, such as a professional or technical component modifier or a therapy modifier.
  • N823 (Incomplete/Invalid procedure modifier(s).): The modifier billed is incomplete or invalid for this code.
  • N519 (Invalid combination of HCPCS modifiers.): The combination of modifiers on the line is not allowed together.
  • N56 (Procedure code billed is not correct/valid for the services billed or the date of service billed.): The procedure code itself may be wrong for what was billed, not only the modifier.
  • CO-182 (Procedure modifier was invalid on the date of service.): The modifier itself was not valid on the date of service, rather than inconsistent with the code.
  • CO-236 (This procedure or procedure/modifier combination is not compatible with another procedure or procedure/modifier combination provided on the same day…): The procedure or procedure/modifier combination conflicts with another procedure under NCCI or payer edits.
  • CO-97 (The benefit for this service is included in the payment/allowance for another service/procedure that has already been adjudicated.): Bundled service, often triggered when a needed modifier such as 59 or 25 is missing.
  • CO-16 (Claim/service lacks information or has submission/billing error(s).): General missing or invalid information; some payers use it with a modifier remark code instead.

CO-4 FAQ

What is the most common reason for CO-4?

A modifier that does not apply to the procedure, such as a professional or technical component modifier on a code with no such split, or a missing required modifier like a therapy or telehealth modifier. The remark codes on the ERA usually narrow it down.

Do I appeal or resubmit a CO-4 denial?

Usually resubmit. If your modifier was wrong or missing, send a corrected claim with resubmission code 7 and the original claim number in box 22. Appeal only if the modifier was correct and supported by payer policy.

Can I bill the patient for a CO-4 denial?

No. Under the CO group code the provider is responsible. Correct the claim instead.

Is CO-4 the same as an NCCI modifier denial?

Not exactly. CO-4 is about the modifier not fitting the procedure code on its own line. NCCI pair edits between two procedures usually come back as CO-236 or CO-97.