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MA66 Remark Code: Principal Procedure Code Invalid

MA66 means the principal procedure code on the claim was missing, incomplete, or invalid. On inpatient institutional claims this is the ICD-10-PCS code for the main procedure performed during the stay.

Quick facts

Code
MA66 (RARC MA66)
Status
Active In use since January 1, 1997; last modified December 2, 2004.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The claim was returned or adjusted because of the coding error. The facility must fix and resubmit it rather than billing the patient.
Official description
Missing/incomplete/invalid principal procedure code.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What MA66 means

MA66 tells you the payer could not accept the principal procedure code on the claim. On inpatient hospital claims, the principal procedure is the ICD-10-PCS code for the procedure performed for definitive treatment, or the one most closely related to the principal diagnosis. It goes in UB-04 form locator 74, with its date alongside. The X12 notes link MA66 to N303, which covers that date.

The remark usually explains CARC 16 or a code-validity reason, and the claim needs a corrected code before it can be paid.

Common causes

  • A procedure was performed but the principal procedure field was left blank.
  • The ICD-10-PCS code has fewer than seven characters or uses an invalid character combination.
  • An outpatient code set was entered where ICD-10-PCS is required.
  • The code was deleted or not yet effective on the procedure date.
  • Two procedures were reported but the one designated principal was not the correct choice.

How to fix it

  1. Review the operative report to identify the principal procedure.
  2. Build the ICD-10-PCS code with all seven characters and validate it for the discharge date.
  3. Report the procedure date in the matching field so the claim does not also trigger N303.
  4. Re-run DRG grouping to confirm the expected payment.
  5. Resubmit as instructed by the payer, using a corrected type-of-bill frequency code if the original claim was processed.

How to prevent it

  • Require coder sign-off on principal procedure selection for surgical stays.
  • Build a claim edit that rejects inpatient claims with a surgical DRG but no principal procedure.
  • Load the new ICD-10-PCS tables each October before discharges on or after the effective date are billed.

For more on why payers return claims like this, see claim rejection vs. denial.

Codes that may appear with MA66

  • CO-16 (Claim/service lacks information or has submission/billing error(s).): Missing information needed for adjudication; MA66 points to the principal procedure field.
  • CO-181 (Procedure code was invalid on the date of service.): The procedure code was invalid on the date of service.
  • N303 (Missing/incomplete/invalid principal procedure date.): The principal procedure date, a companion field that must also be present and valid.
  • MA63 (Missing/incomplete/invalid principal diagnosis.): The principal diagnosis, which is often reviewed together with the principal procedure.
  • M51 (Missing/incomplete/invalid procedure code(s).): Missing or invalid procedure codes in general, not specifically the principal one.

MA66 FAQ

Which claims need a principal procedure code?

Inpatient hospital claims where a significant procedure was performed. It is reported in UB-04 form locator 74 using ICD-10-PCS.

Can I report a HCPCS code as the principal procedure?

Not on an inpatient claim. Inpatient procedures use ICD-10-PCS, while HCPCS and similar codes are reported on service lines for outpatient billing.

Does fixing MA66 change the DRG?

It can. The principal procedure feeds DRG grouping, so correcting it may change reimbursement. Re-run the grouper before resubmitting.