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M91 Remark Code: Split Claims by CLIA Number

M91 means laboratory procedures performed under different CLIA certification numbers were billed together on one claim. The payer requires a separate claim for each CLIA number, so you need to split the claim and resubmit.

Quick facts

Code
M91 (RARC M91)
Status
Active In use since January 1, 1997.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The lab lines were not paid because the claim structure was wrong. The provider corrects and rebills; the patient is not charged.
Official description
Lab procedures with different CLIA certification numbers must be billed on separate claims.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What M91 means

Laboratories are certified under the Clinical Laboratory Improvement Amendments (CLIA), and each certified site has its own CLIA number. Payers such as Medicare check that the number on a claim covers the tests billed. Because professional claims carry a single CLIA number at the claim level, a claim that includes tests from two certified sites cannot be validated. M91 tells you to split it.

This is a structural billing issue. The tests may be fully covered once they are billed on the right claims.

Common causes

  • A multi-site practice billing tests from its main lab and a satellite lab on one claim.
  • Office tests performed under a certificate of waiver combined with tests sent to a separate certified lab the practice bills for.
  • A billing system that uses one default CLIA number for all lab lines.
  • Referred lab tests mixed in with in-house tests.

How to fix it

  1. Group the lab lines by the site and CLIA certificate that performed each test.
  2. Create one claim per CLIA number, entering the correct number for each.
  3. Confirm each test is within the scope of the certificate on its claim, such as waived tests on a waiver certificate.
  4. Submit the new claims. If the original was partly adjudicated, follow the payer’s instructions for corrected or replacement claims so nothing is paid twice.

How to prevent it

  • Map each lab test in the charge master to the CLIA number of the site that performs it.
  • Configure claim creation to split automatically by CLIA number.
  • Review the CO-16 guide for other claim-structure rejections.

Codes that may appear with M91

  • CO-16 (Claim/service lacks information or has submission/billing error(s).): The claim has a submission or billing error, identified by M91 as mixed CLIA numbers.
  • MA120 (Missing/incomplete/invalid CLIA certification number.): The CLIA number itself is missing, incomplete, or invalid.
  • N62 (Dates of service span multiple rate periods.): Another instruction to split a claim, in that case because dates span multiple rate periods.

M91 FAQ

Why can a claim only carry one CLIA number?

On professional claims, the CLIA number is reported once at the claim level (for example in box 23 of the CMS-1500), so the payer cannot match lines to different certificates on the same claim.

When would a practice have more than one CLIA number?

When tests are performed at different lab sites, such as an office lab with a waiver certificate and a separate certified lab, or when a practice bills purchased or referred tests.

Do I appeal M91?

No. Split the claim and submit new or corrected claims, one per CLIA number.