CO 146 denial code

Claim adjustment reason code 146. Often reported as CO 146.

Diagnosis invalid for the date of service

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 146 means

The diagnosis code on the claim was not a valid code on the date the service was provided.

Diagnosis codes are added, deleted and expanded each year. This code means the one you reported did not exist, or was not valid to that level of detail, on the date of service.

Usually the fix is coding, not appealing: find the code that was valid on that date, at full specificity, and send a corrected claim.

Group codes seen with 146

On a remittance, 146 comes after a group code. These are the ones it is most often reported with:

• CO 146 (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

• A deleted diagnosis code was used after its end date.
• A new diagnosis code was billed for a date before it took effect.
• The code was not reported to the full number of characters required.

What to do next

1. Look up the diagnosis code in the code set that was in effect on the date of service.
2. Replace it with the valid code at full specificity, confirmed against the medical record.
3. Send a corrected claim and update your system's code tables if they are out of date.

Related codes

• 181 (reason code) Procedure code invalid on date of service
• 16 (reason code) Missing information or billing error
• 167 (reason code) Diagnosis not covered