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M120 Remark Code (Deactivated): Substitute Physician ID

M120 meant the identifier for the substituting physician who provided services under a reciprocal billing or locum tenens arrangement was missing, incomplete, or invalid. X12 deactivated it without naming a replacement.

X12 deactivated RARCM120 on June 2, 2005. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Code: M120 (RARC M120)
Status: Deactivated StoppedJune 2, 2005 (in use since January 1, 1997).
Code set: Remittance Advice Remark Codes (RARC)
Group codes: CO (Contractual Obligation): Accompanied CO missing-information denials; the billing physician needed to report the substitute correctly.
Official description: Missing/incomplete/invalid provider identifier for the substituting physician who furnished the service(s) under a reciprocal billing or locum tenens arrangement.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What M120 meant

When a physician is absent, a substitute may treat their patients, and under reciprocal billing or locum tenens rules the regular physician can bill for those services. Payers still need to know who actually provided the care. Remark M120 said the substitute physician’s identifier was missing or invalid.

What replaced it

X12 did not name a successor. Current claims identify providers by NPI, and payer instructions determine where the substitute’s NPI goes. If the rendering provider identifier is the problem, payers generally use N290, with CARC CO-16. Errors in the substitute-billing modifiers can draw N823.

If you still see M120

It will only be in older claims. For current substitute-physician billing, follow the payer’s rules on how long a substitute may be used, which modifier applies, and where the substitute’s NPI goes. Keep a record of each arrangement and its dates.

N290 (Missing/incomplete/invalid rendering provider primary identifier.): Missing, incomplete, or invalid rendering provider primary identifier.
CO-16 (Claim/service lacks information or has submission/billing error(s).): Claim lacks information or has billing errors.
N823 (Incomplete/Invalid procedure modifier(s).): Incomplete or invalid procedure modifier, relevant when substitute-billing modifiers are wrong.

M120 FAQ

What are reciprocal billing and locum tenens arrangements?

They let a regular physician bill for services provided by a substitute physician while the regular physician is away, under conditions set by the payer. Medicare, for example, has specific rules and uses HCPCS modifiers Q5 and Q6 for these arrangements.

How is the substitute identified today?

Payer rules vary. Medicare has the regular physician bill the services with modifier Q5 or Q6 and keep a record of each service with the substitute's NPI, available to the contractor on request. Other payers may want the substitute's identifier on the claim, so check their current instructions.

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