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M100 Remark Code: Oral Anti-Emetic Timing Not Met

M100 means the payer does not cover an oral anti-emetic drug unless it is administered for use immediately before, at, or within 48 hours of a covered chemotherapy drug. The claim did not meet that timing condition.

Quick facts

Code
M100 (RARC M100)
Status
Active In use since January 1, 1997.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The drug is denied under coverage rules, and the supplier absorbs it unless a valid patient agreement exists.
  • PR (Patient Responsibility): The patient is responsible, typically when an advance notice of non-coverage was obtained.
Official description
We do not pay for an oral anti-emetic drug that is not administered for use immediately before, at, or within 48 hours of administration of a covered chemotherapy drug.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What M100 means

Medicare Part B covers certain oral anti-emetic drugs when they are used as part of a cancer chemotherapy regimen, specifically for use immediately before, at, or within 48 hours after administration of a covered chemotherapy drug. M100 tells you that the payer could not confirm that link, so it denied the anti-emetic.

This is a coverage condition, not a formatting problem. To get paid, the claim and records must show the relationship to covered chemotherapy.

Common causes

  • The dispensing or dates billed extend beyond the 48-hour window.
  • The chemotherapy drug is not a covered drug under the payer’s policy, or was not billed.
  • The claim did not include the diagnosis or indicators the payer uses to connect the drug to chemotherapy.
  • Anti-emetics used for other reasons billed under the chemotherapy benefit.

How to fix it

  1. Confirm the chemotherapy date and drug from the treating provider.
  2. Check that the anti-emetic quantity and dates fit within the covered window.
  3. Correct the claim if dates, quantities, or diagnoses were wrong, and resubmit as a corrected claim with frequency code 7.
  4. Appeal with records if the timing was met but not recognised.
  5. Look for other coverage such as a prescription drug plan for doses outside the window.

How to prevent it

  • Dispense and bill anti-emetics in quantities aligned with the chemotherapy schedule.
  • Get the chemotherapy date on every order.
  • Review authorization and coverage prevention for building coverage checks into intake.

Codes that may appear with M100

  • CO-96 (Non-covered charge(s).): The anti-emetic is non-covered, with M100 stating the timing condition.
  • CO-50 (These are non-covered services because this is not deemed a 'medical necessity' by the payer.): The payer found the drug not medically necessary as billed.
  • CO-B15 (This service/procedure requires that a qualifying service/procedure be received and covered.): The qualifying chemotherapy service was not found or not covered.
  • M119 (Missing/incomplete/invalid/ deactivated/withdrawn National Drug Code (NDC).): The NDC for the drug was missing or invalid, which is a data issue rather than a timing issue.
  • N386 (This decision was based on a National Coverage Determination (NCD).): The decision was based on a National Coverage Determination.

M100 FAQ

Why does the 48-hour window matter?

Medicare covers certain oral anti-emetics as a replacement for intravenous anti-emetics given with chemotherapy. Coverage is tied to use around the time of a covered chemotherapy drug.

What if the chemotherapy was billed by another provider?

The payer still needs to link the anti-emetic to covered chemotherapy. Documentation of the chemotherapy date and drug helps when that claim is from someone else.

Could the drug be covered under another benefit?

Possibly, such as a prescription drug plan, depending on the patient's coverage. Check before billing the patient.