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M8 Remark Code: Blood Gas Test Not Acceptable

M8 means the payer will not accept the blood gas test results used to justify the claim, usually for home oxygen, because the test was performed by a medical supplier or was taken while the patient was already on oxygen.

Quick facts

Code
M8 (RARC M8)
Status
Active In use since January 1, 1997.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The supplier absorbs the denied amount because the qualifying test did not meet the payer's requirements.
  • PR (Patient Responsibility): The patient may be liable only if a valid advance beneficiary notice was obtained before the equipment was furnished.
Official description
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.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What M8 means

Home oxygen coverage depends on objective evidence that the patient needs it, typically an arterial blood gas or oximetry result. M8 says the payer rejected that evidence for one of two reasons stated in the official text:

  • The test was conducted by a medical supplier, which creates a conflict of interest when the supplier also bills for the oxygen.
  • The test was taken while the patient was on oxygen, so the result did not reflect the condition the payer’s criteria require.

This is a Medicare DMEPOS remark in origin. The denial itself comes through a reason code such as CARC 50.

Common causes

  • The oxygen supplier or its staff performed the oximetry test.
  • The testing entity is affiliated with the supplier.
  • The test report does not state the conditions (rest, exercise, sleep, room air or oxygen) and appears to have been taken on oxygen.
  • The ordering provider submitted an old test that did not meet the criteria.

How to fix it

  1. Review the test report for who performed it and under what conditions.
  2. Arrange a new qualifying test through an independent qualified provider if the patient still needs oxygen.
  3. Update the certification or order with the new result.
  4. Submit a corrected claim or reopening for eligible months, following the payer’s instructions, or file an appeal if you believe the original test met the rules.

How to prevent it

Never let supplier staff perform or arrange qualifying tests under the supplier’s own name. Check each oxygen referral for who ran the test and the testing conditions before delivery. A pre-submission review with the Claims Validator can flag missing oxygen documentation fields.

Codes that may appear with M8

  • CO-50 (These are non-covered services because this is not deemed a 'medical necessity' by the payer.): Oxygen was not shown to be medically necessary because the qualifying test was not acceptable.
  • CO-16 (Claim/service lacks information or has submission/billing error(s).): The claim lacked a valid qualifying test result or test information.
  • M19 (Missing oxygen certification/re-certification.): The oxygen certification or recertification itself is missing.
  • N750 (Incomplete/invalid Blood Gas Report.): The blood gas report is incomplete or invalid, rather than performed by the wrong party.
  • N328 (Missing/incomplete/invalid Oxygen Saturation Test date.): The oxygen saturation test date is missing or invalid.

M8 FAQ

Who is allowed to perform the qualifying blood gas test?

Under Medicare rules, the test must be performed by a qualified provider or laboratory, not by the DME supplier that furnishes the oxygen or a related entity. Other payers may have their own standards.

Can I resubmit with a new test?

If a new qualifying test was performed properly within the time frame the payer requires, you can submit updated documentation. The original test cannot be reused.

Does M8 affect claims for oxygen already delivered?

Yes. Months billed on the strength of the rejected test are typically denied until an acceptable qualifying test supports coverage.