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M64 Remark Code: Secondary Diagnosis Invalid

M64 means a secondary or 'other' diagnosis on the claim is missing, incomplete, or invalid. The payer needed a valid additional diagnosis to process the claim or line.

Quick facts

Code
M64 (RARC M64)
Status
Active In use since January 1, 1997; last modified February 28, 2003.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The line is unpaid until the diagnosis is corrected. The patient is not billed for coding errors.
Official description
Missing/incomplete/invalid other diagnosis.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What M64 means

Claims can list many diagnoses. The first is the primary or principal diagnosis; the rest are secondary, or “other,” diagnoses. M64 says one of those secondary codes did not pass the payer’s validation. It might be truncated, retired, not yet effective, or referenced by a pointer that leads to nothing.

It is usually paired with CARC 16 or 146.

Common causes

  • An ICD-10-CM code missing its final character or seventh character extension.
  • A code deleted in the annual update still used after its end date.
  • A diagnosis pointer in box 24E referencing a blank line in box 21.
  • Decimal points or extra characters in electronic claims.
  • A header code used where a billable code was required.

How to fix it

  1. Identify the flagged diagnosis from the remittance or rejection report.
  2. Look it up in the ICD-10-CM code set effective for the date of service.
  3. Correct box 21 (or form locators 67A-Q) and check each pointer in box 24E.
  4. Resubmit using resubmission code 7 or a new claim, per the payer’s instructions.

How to prevent it

Load ICD-10-CM updates each October and April, and run diagnosis validation on every claim before submission. Front-end checks with the Claims Validator help find truncated codes. See the CO-16 guide.

Codes that may appear with M64

  • CO-16 (Claim/service lacks information or has submission/billing error(s).): The claim contains missing information or billing errors.
  • CO-146 (Diagnosis was invalid for the date(s) of service reported.): The diagnosis was invalid for the date of service.
  • CO-11 (The diagnosis is inconsistent with the procedure.): The diagnosis is inconsistent with the procedure.
  • MA63 (Missing/incomplete/invalid principal diagnosis.): The principal diagnosis, not a secondary one, is missing or invalid.
  • M76 (Missing/incomplete/invalid diagnosis or condition.): A diagnosis or condition is missing or invalid, without specifying which position.
  • M81 (You are required to code to the highest level of specificity.): The diagnosis was not coded to the highest level of specificity.

M64 FAQ

What counts as an 'other' diagnosis?

Any diagnosis beyond the first, or principal, one. On the CMS-1500 these are lines B through L in box 21. On the UB-04 they are in form locators 67A through 67Q.

Can an incomplete code trigger M64?

Yes. ICD-10-CM codes must be reported to their full required length. A code missing a required character is invalid.

What about diagnosis pointers?

If box 24E points to a diagnosis line that is empty or invalid, the payer may flag the secondary diagnosis.