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CO-A8 Denial Code: Ungroupable DRG

CO-A8 means the inpatient claim couldn't be assigned to a diagnosis-related group (DRG). The grouper software rejected the combination of diagnoses, procedures, demographics, or discharge data, so the claim can't be priced until the data is corrected.

Quick facts

Code
CO-A8 (CARC A8)
Status
Active In use since January 1, 1995; last modified September 30, 2007.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The usual group. The facility is responsible for correcting the claim data; the patient isn't billed.
Official description
Ungroupable DRG.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-A8 means

CARC A8 says ungroupable DRG. Inpatient claims paid on a DRG basis go through grouper software, which reads the principal diagnosis, secondary diagnoses, procedures, age, sex, discharge status, and (for newborns) birth weight, then assigns a diagnosis-related group. If the data can’t produce a valid DRG, the payer can’t price the claim and denies it with A8.

Under Medicare’s MS-DRG system, these claims end up in special groups: MS-DRG 998 when the principal diagnosis isn’t valid as a discharge diagnosis, and MS-DRG 999 when the claim is otherwise ungroupable. Other payers using DRG methods, including APR-DRG, have similar outcomes.

This is an institutional code, and it’s almost always a data problem the facility can fix.

Example: a claim lists a symptom code that the grouper doesn’t accept as a principal diagnosis. The grouper assigns an ungroupable DRG, and the remittance returns CO-A8. The coder reviews the record, sequences the definitive diagnosis correctly, and the corrected claim groups normally.

Common causes

  • Invalid principal diagnosis, such as a code not acceptable as a principal discharge diagnosis.
  • Codes not valid on the discharge date, such as deleted codes or incomplete codes lacking required characters.
  • Age or sex conflicts with diagnosis or procedure codes.
  • Invalid discharge status code.
  • Missing birth weight or other required data for newborn claims.
  • Grouper version mismatch between the facility and the payer for the discharge date.

How to fix it

  1. Regroup the claim internally using the grouper version for the discharge date.
  2. Identify the failing edit, such as principal diagnosis, code validity, demographics, or discharge status.
  3. Review the medical record and correct coding or sequencing as documentation supports.
  4. Submit a corrected institutional claim through the payer’s replacement process.
  5. Confirm the payer’s grouper version if your result differs from theirs.

How to prevent it

  • Run pre-bill DRG validation on every inpatient claim.
  • Load annual code and grouper updates on time for the new fiscal year.
  • Validate demographics and discharge status before final billing.
  • Scrub claims for invalid codes. A Claims Validator can catch deleted or incomplete codes before release.

Specialty notes

Hospitals of all sizes can see A8 after annual ICD-10 updates. Newborn and obstetric claims are sensitive to birth weight, age, and sex edits, which makes them common sources of ungroupable claims.

Remark codes that may appear with CO-A8

  • N208 (Missing/incomplete/invalid DRG code.): The DRG code is missing, incomplete, or invalid.
  • M76 (Missing/incomplete/invalid diagnosis or condition.): A diagnosis is missing, incomplete, or invalid.
  • MA43 (Missing/incomplete/invalid patient status.): The patient discharge status is missing or invalid, which affects grouping.
  • CO-16 (Claim/service lacks information or has submission/billing error(s).): Missing or invalid claim information more generally.
  • CO-146 (Diagnosis was invalid for the date(s) of service reported.): The diagnosis was invalid for the date of service.
  • CO-9 (The diagnosis is inconsistent with the patient's age.): Diagnosis inconsistent with age, one of the edits that can prevent grouping.
  • CO-70 (Cost outlier - Adjustment to compensate for additional costs.): Cost outlier, which can't be calculated until the claim groups.

CO-A8 FAQ

What makes a DRG ungroupable?

Common reasons are a principal diagnosis that isn't valid as a discharge diagnosis, invalid or deleted ICD-10-CM or ICD-10-PCS codes for the discharge date, age or sex conflicts, invalid discharge status, or missing birth weight on newborn claims where required.

Which MS-DRGs indicate an ungroupable claim?

Under Medicare's MS-DRG system, claims that can't be grouped fall into special DRGs such as 998 (principal diagnosis invalid as discharge diagnosis) and 999 (ungroupable).

How do I fix CO-A8?

Run the claim through your own grouper for the discharge date, find the edit that fails, correct the coding or data, and submit a corrected institutional claim.

Does CO-A8 apply to outpatient claims?

It's specific to DRG-based inpatient payment. Outpatient claims have different grouping systems and edits.