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M19 Remark Code: Oxygen Certification Missing

M19 means the payer did not receive the required oxygen certification or recertification for this claim. Without it, home oxygen cannot be paid.

Quick facts

Code
M19 (RARC M19)
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 supplier absorbs the amount until the certification is supplied. The patient is not billed for missing paperwork.
Official description
Missing oxygen certification/re-certification.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What M19 means

Home oxygen is one of the most documentation-heavy DME benefits. The payer needs proof that the patient meets the coverage criteria, usually through a certification that includes qualifying test results and the prescriber’s order. M19 means no certification or recertification was on file or attached for the period billed.

This is a Medicare DMEPOS remark at its core. Its sibling N234 covers certifications that exist but are incomplete or invalid.

Common causes

  • The initial certification was never submitted with the first claim.
  • A recertification was due and not sent.
  • The certification was sent under a different patient identifier or supplier number.
  • The electronic claim did not include the required certification segment.
  • The prescriber did not sign or return the form in time.

How to fix it

  1. Check the certification status with the DME MAC or payer portal.
  2. Obtain the completed, signed certification from the prescriber, with qualifying test data.
  3. Submit it as directed by the payer, then resubmit the claim, typically as a new claim if the original was unprocessable.
  4. Review every month billed without certification and resubmit those too.

How to prevent it

Track certification and recertification due dates per patient and block oxygen claims when certification is missing. See the CO-16 guide for more on missing-data denials.

Codes that may appear with M19

  • CO-16 (Claim/service lacks information or has submission/billing error(s).): The claim lacks information needed for adjudication, here the oxygen certification.
  • CO-252 (An attachment/other documentation is required to adjudicate this claim/service.): An attachment or other documentation is required.
  • CO-50 (These are non-covered services because this is not deemed a 'medical necessity' by the payer.): Without certification, medical necessity for oxygen is not established.
  • N234 (Incomplete/invalid oxygen certification/re-certification.): The certification is present but incomplete or invalid, rather than missing.
  • M8 (We do not accept blood gas tests results when the test was conducted by a medical supplier or taken while the patient is on oxygen.): The qualifying blood gas test was not acceptable.
  • N328 (Missing/incomplete/invalid Oxygen Saturation Test date.): The oxygen saturation test date is missing or invalid.

M19 FAQ

What counts as oxygen certification?

For Medicare, the supplier has historically submitted a certificate of medical necessity (CMN) or the payer's required information set, with the qualifying test results and the prescriber's order. Requirements have changed over time, so check the current DME MAC instructions.

When is recertification needed?

The payer defines when a recertification or new certification is required, for example after a break in need or when test results or orders change.

Can I bill the patient while waiting for the certification?

No. M19 is a supplier documentation issue, not patient liability.