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CO-D21 Denial Code (Deactivated): Diagnosis Missing/Invalid

CO-D21 meant one or more diagnoses on the claim were missing or invalid. X12 deactivated it without naming a replacement; payers now typically use CARC 16 with remark M76, or CARC 146 for diagnoses invalid on the date of service.

X12 deactivated CARCD21 on June 30, 2007. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Code: CO-D21 (CARC D21)
Status: Deactivated StoppedJune 30, 2007 (in use since January 1, 1995).
Code set: Claim Adjustment Reason Codes (CARC)
Group codes: CO (Contractual Obligation): Correct diagnosis coding is the provider's responsibility; the patient was not billed.
Official description: This (these) diagnosis(es) is (are) missing or are invalid
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What CO-D21 meant

CARC D21 addressed the quality of diagnosis codes. Either a diagnosis was absent where one was needed, or the code reported could not be accepted, for instance because it was truncated or not valid in the code set. It differed from coverage denials, which say a valid diagnosis is not covered.

What replaced it

X12 did not name a replacement. The active codes that cover this ground are CO-16 with remark M76 for a missing or invalid diagnosis, and CO-146 when the diagnosis was not valid on the service date. Payers may add M81 when the code is not specific enough.

If you still see CO-D21

It appears only in historical claims. When correcting a similar denial today, validate each diagnosis against the code set effective on the date of service, add missing characters where the documentation supports them, and make sure every service line points to a valid diagnosis. Then submit a corrected claim.

CO-16 (Claim/service lacks information or has submission/billing error(s).): Claim lacks information or has billing errors, used with a remark code.
CO-146 (Diagnosis was invalid for the date(s) of service reported.): Diagnosis was invalid for the date of service reported.
M76 (Missing/incomplete/invalid diagnosis or condition.): Remark code for a missing, incomplete, or invalid diagnosis or condition.
M81 (You are required to code to the highest level of specificity.): Remark code: you are required to code to the highest level of specificity.

CO-D21 FAQ

What makes a diagnosis code invalid?

It may not exist in the code set in effect on the date of service, it may be a category header rather than a billable code, or it may lack required characters.

Was CO-D21 a medical necessity denial?

No. It concerned whether the diagnosis was present and valid, not whether it justified the service. Coverage of a diagnosis is a separate question.

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