CO 9 denial code

Claim adjustment reason code 9. Often reported as CO 9.

Diagnosis does not fit the patient's age

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 9 means

One of the diagnosis codes on the claim is not valid for the patient's age, according to the payer's edits.

Some diagnosis codes are limited to certain ages, such as perinatal, pediatric or maternity conditions. The payer found a diagnosis that does not fit the age it calculated from the date of birth.

Check both the diagnosis and the date of birth, since either one can be the problem. If a payer policy number appears at the line level, pull that policy first to see what it requires.

Group codes seen with 9

On a remittance, 9 comes after a group code. These are the ones it is most often reported with:

• CO 9 (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 date of birth on the claim or on the payer's file is wrong.
• A perinatal or newborn diagnosis was used for a patient past the age range it covers.
• A diagnosis code was picked from a list without checking the age guidance in the code set.

What to do next

1. Confirm the patient's date of birth with registration and the payer's eligibility record.
2. Review the diagnosis code against the code set guidance for age limits.
3. Send a corrected claim with the right diagnosis or demographics.
4. Appeal with documentation if the diagnosis is valid and the payer edit is too strict.

Related codes

• 6 (reason code) Code does not fit the patient's age
• 11 (reason code) Diagnosis does not support the procedure
• 16 (reason code) Missing information or billing error