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CO-231 Denial Code: Mutually Exclusive Procedures

CO-231 means the payer found two procedures that are mutually exclusive, meaning they cannot reasonably be done on the same patient in the same day or setting. One is denied. A modifier is only appropriate if the services truly were separate and distinct.

Quick facts

Code
CO-231 (CARC 231)
Status
Active In use since July 1, 2009; last modified July 1, 2017.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The provider is responsible for the denied procedure, which generally cannot be billed to the patient.
Official description
Mutually exclusive procedures cannot be done in the same day/setting. 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-231 means

CARC 231 says mutually exclusive procedures cannot be done in the same day or setting. The payer’s editing found two codes on the claim (or on claims for the same date) that describe services that should not both happen at the same encounter. It pays one and denies the other with CO-231.

Mutually exclusive edits are part of the NCCI procedure-to-procedure (PTP) edit tables Medicare publishes, and most commercial payers apply similar logic. Typical pairs are two procedures that describe different methods for the same thing, or an initial and a repeat version of a service on the same day. The official usage points to the 835 Healthcare Policy Identification REF in loop 2110, where the payer may cite the specific policy.

Common causes

  • Coding two methods for the same procedure, when only one was actually done or the documentation supports only one.
  • Coding a procedure and its converted or abandoned version, where only the final one should be billed.
  • Distinct procedures without a modifier, such as different sites or separate sessions, missing modifier 59 or an X modifier (XE, XS, XP, XU) in box 24D.
  • Two providers in the same group billing overlapping procedures for the same patient on the same date.
  • Split claims where the second claim contains the mutually exclusive code.

How to fix it

  1. Look up the edit pair in the current NCCI PTP tables or the payer’s policy for the date of service, and note whether a modifier is allowed.
  2. Review the documentation. Was the second procedure actually performed separately, at a different site or session?
  3. If only one was performed or both were described by one code, accept the denial or submit a corrected claim (frequency code 7) with the right code.
  4. If both were distinct and the edit allows a modifier, submit a corrected claim with the appropriate modifier on the correct line and make sure the notes support it.
  5. If you believe the edit was misapplied, file a reconsideration with the operative or procedure note.
  6. Do not bill the patient for the denied procedure under the CO group.

See NCCI denials, PTP bundling, and modifiers for a full explanation of edit pairs and modifier indicators.

How to prevent it

  • Load current NCCI PTP edits into your billing software and update them quarterly.
  • Train coders on mutually exclusive pairs common in your specialty.
  • Require documentation of separate site or session before a distinct-service modifier is added.
  • Run claims through a Claims Validator that flags PTP pairs before submission.
  • Coordinate same-day billing between providers in the same group.

Remark codes that may appear with CO-231

  • M80 (Not covered when performed during the same session/date as a previously processed service for the patient.): The service is not covered when performed in the same session or date as a previously processed service.
  • N20 (Service not payable with other service rendered on the same date.): The service is not payable with another service rendered on the same date.
  • N519 (Invalid combination of HCPCS modifiers.): The combination of HCPCS modifiers submitted is invalid.
  • CO-236 (This procedure or procedure/modifier combination is not compatible with another procedure or procedure/modifier combination provided on the same day…): A procedure or modifier combination is incompatible with another on the same day under NCCI or payer policy.
  • CO-97 (The benefit for this service is included in the payment/allowance for another service/procedure that has already been adjudicated.): The service is bundled into another service's payment, rather than mutually exclusive with it.
  • CO-234 (This procedure is not paid separately.): The procedure is not paid separately.
  • CO-59 (Processed based on multiple or concurrent procedure rules. (For example multiple surgery or diagnostic imaging, concurrent anesthesia.)): Payment was reduced under multiple-procedure rules, not denied.

CO-231 FAQ

What does mutually exclusive mean in coding?

Two procedures are mutually exclusive when they generally would not be performed on the same patient at the same session, for example two different approaches to the same procedure. NCCI procedure-to-procedure edits include many such pairs.

Can I add modifier 59 to fix CO-231?

Only if documentation shows the procedures were truly distinct, such as a different session, site, or organ system, and the edit allows a modifier. Adding 59 or an X modifier just to bypass the edit is not appropriate.

Which procedure does the payer pay?

Payers usually pay the column one code of the NCCI edit pair, or the higher-valued procedure, and deny the other. Check the payer's policy for how it chooses.