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MA51 Remark Code (Deactivated): Office Lab CLIA Number

MA51 meant the CLIA certification number was missing, incomplete, or invalid for laboratory services billed by a physician office laboratory. X12 deactivated it and suggested MA120, which applies to any CLIA number problem.

X12 deactivated RARCMA51 on February 5, 2005. Payers should no longer use it on new remittances, but it can still appear on older ERAs, corrected claims, and appeals.

Code: MA51 (RARC MA51)
Status: Deactivated StoppedFebruary 5, 2005 (in use since January 1, 1997).
Code set: Remittance Advice Remark Codes (RARC)
Group codes: CO (Contractual Obligation): Paired with CO adjustments; the office had to supply its CLIA number.
Official description: Missing/incomplete/invalid CLIA certification number for laboratory services billed by physician office laboratory.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What MA51 meant

Physician office laboratories need CLIA certification to perform tests, even simple ones. Remark MA51 told an office that its lab claim lacked a valid CLIA number, so the payer could not confirm the office was certified for the tests billed.

What replaced it

X12’s note suggested MA120. MA120 is active and covers a missing, incomplete, or invalid CLIA number regardless of the type of lab. When the number is valid but the certificate does not permit the test, payers use CARC CO-B23. Missing-number denials usually carry CO-16.

If you still see MA51

It is found only in older remittances. For a current CLIA rejection, confirm the certificate is active, check that its type covers each test billed, and enter the number in the field your payer specifies before resubmitting.

MA120 (Missing/incomplete/invalid CLIA certification number.): The suggested replacement: missing, incomplete, or invalid CLIA certification number.
CO-B23 (Procedure billed is not authorized per your Clinical Laboratory Improvement Amendment (CLIA) proficiency test.): Procedure billed is not authorized per the CLIA proficiency test.
CO-16 (Claim/service lacks information or has submission/billing error(s).): Claim lacks information or has billing errors.

MA51 FAQ

How was MA51 different from M34?

M34 said a claim lacked the CLIA number in general. MA51 applied specifically to physician office laboratories and also covered numbers that were incomplete or invalid.

Do waived tests need a CLIA number?

Payers commonly require the CLIA number on claims for waived tests too, because the office still needs a certificate of waiver to perform them.

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