CO 10 denial code

Claim adjustment reason code 10. Often reported as CO 10.

Diagnosis does not fit the patient's sex

Unofficial reference. This is our own plain-English explanation, not the official code text. Payer rules differ. Verify with the payer and the official X12 code list. AlphaPrefix is not affiliated with X12, CMS or any payer. Data last updated Nov 11, 2026.

What 10 means

A diagnosis code on the claim conflicts with the patient's sex as recorded on the claim or by the payer.

Certain diagnoses are tied to one sex in payer edit tables. When the claim or the payer's member file shows the other sex, the line is denied with this code.

As with procedure edits, the patient's record may not match the diagnosis for valid reasons, and payers often have a specific way to report that. A policy reference at the service line, when the payer sends one, names the rule behind the decision.

Group codes seen with 10

On a remittance, 10 comes after a group code. These are the ones it is most often reported with:

• CO 10 (Contractual Obligation): The provider's side of the ledger. The reduction comes from the payer contract or a rule the provider has to follow, and the patient normally cannot be billed for it.

The payer chooses the group code for each adjustment, so the same reason code can appear with another group code on your remittance. Read the two together. More about group codes.

Common causes

• The patient's sex was entered incorrectly on the claim or at registration.
• The payer's member file shows a different sex than the practice's record.
• The diagnosis is correct for the patient, but the payer edit does not allow it without a specific indicator.

What to do next

1. Compare the sex on the claim with registration and the payer's eligibility response.
2. Correct the demographics or the diagnosis and send a corrected claim.
3. Follow the payer's instructions for reporting diagnoses that do not match the sex on file.
4. Appeal with medical records if the claim was correct.

Related codes

• 7 (reason code) Code does not fit the patient's sex
• 9 (reason code) Diagnosis does not fit the patient's age
• 16 (reason code) Missing information or billing error