CO A8 denial code

Claim adjustment reason code A8. Often reported as CO A8.

DRG could not be grouped

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 A8 means

The payer's grouper software could not assign a DRG to the inpatient claim, so it could not be priced.

Inpatient claims paid by DRG depend on a valid combination of diagnoses, procedures, discharge status, age and sex. When that combination fails the grouper logic, the claim lands in an ungroupable category and the payer reports A8.

This is almost always a coding or data problem on the claim, so the answer is to find and fix the field that broke the grouping.

Group codes seen with A8

On a remittance, A8 comes after a group code. These are the ones it is most often reported with:

• CO A8 (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 principal diagnosis is invalid, too vague, or not acceptable as a principal diagnosis.
• The patient's age, sex or discharge status conflicts with the reported diagnosis or procedure codes.
• Codes were invalid for the discharge date because of an annual code set update.

What to do next

1. Run the claim through your own grouper or encoder to see where grouping fails.
2. Review the principal diagnosis, procedure codes, discharge status and demographic fields with coding staff.
3. Submit a corrected claim once the coding or data error is fixed.

Related codes

• A5 (reason code) Medicare PPS capital cost outlier
• 16 (reason code) Missing information or billing error