CO 6 denial code

Claim adjustment reason code 6. Often reported as CO 6.

Code 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 6 means

The payer flagged the procedure or revenue code as not appropriate for the patient's age on file.

Many codes are defined for a certain age group, such as newborn, pediatric or adult services, and payers run edits comparing them with the date of birth they hold. This code means that edit failed.

Often the code is fine and the date of birth is wrong somewhere. Group code CO is typical. When the payer includes a policy reference on the line, read that policy before correcting or appealing.

Group codes seen with 6

On a remittance, 6 comes after a group code. These are the ones it is most often reported with:

• CO 6 (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 date of birth was keyed incorrectly at registration or on the claim.
• The payer has a different date of birth on file than the practice does.
• An age specific code was billed for a patient outside its age range, such as a pediatric vaccine code for an adult.
• A newborn was billed under the parent's identifiers or date of birth.

What to do next

1. Compare the date of birth on the claim with the patient's insurance card and the payer's eligibility response.
2. Correct the demographics and send a corrected claim if the date of birth was wrong.
3. If the date of birth is right, review whether a different, age appropriate code should have been used.
4. Ask the patient to update their date of birth with the plan if the payer's record is wrong.

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