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M101 Remark Code (Deactivated): Report G1-G5 Modifier

M101 warned that the provider should begin reporting a G1 through G5 modifier with this HCPCS code, because the payer would soon deny the service without one. X12 deactivated it and suggested M78, which has also been retired; missing modifiers are now flagged with N822.

X12 deactivated RARCM101 on January 31, 2004. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Code: M101 (RARC M101)
Status: Deactivated StoppedJanuary 31, 2004 (in use since January 1, 1997).
Code set: Remittance Advice Remark Codes (RARC)
Group codes: CO (Contractual Obligation): Sent as a notice under CO; later denials for the missing modifier were provider responsibility.
Official description: Begin to report a G1-G5 modifier with this HCPCS. We will soon begin to deny payment for this service if billed without a G1-G5 modifier.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What M101 meant

Remark M101 was advance notice for dialysis billing. The G1 through G5 modifiers report ranges of a patient’s urea reduction ratio. The payer was telling providers to start attaching one of those modifiers to the relevant HCPCS code, and that claims without it would soon be denied.

What replaced it

X12’s note suggested M78, a general missing or invalid modifier remark. M78 has since been retired. Today, payers use N822 for a missing modifier or N823 for an invalid one, often with CARC CO-4.

If you still see M101

It is only found in old dialysis remittances. For current claims, make sure your dialysis billing captures the adequacy values each month and maps them to the modifier the payer requires, then resubmit anything denied for a missing modifier.

M78Deactivated (Missing/incomplete/invalid HCPCS modifier.): The suggested replacement for a missing or invalid HCPCS modifier, later deactivated.
N822 (Missing procedure modifier(s).): Missing procedure modifier.
N823 (Incomplete/Invalid procedure modifier(s).): Incomplete or invalid procedure modifier.
CO-4 (The procedure code is inconsistent with the modifier used.): The procedure code is inconsistent with the modifier used.

M101 FAQ

What are the G1 through G5 modifiers?

They are HCPCS modifiers used on dialysis claims to report the range of the patient's most recent urea reduction ratio, a measure of dialysis adequacy.

Did M101 deny the claim?

No. It was a warning that future claims would be denied if the modifier was missing.

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