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M45 Remark Code: Occurrence Code Missing or Invalid

M45 means the payer found an occurrence code on the claim missing, incomplete, or invalid. Occurrence codes pair a significant event with its date, and the payer needed that information to process the claim.

Quick facts

Code
M45 (RARC M45)
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 is unpaid until the occurrence code is fixed. It is a provider data issue.
Official description
Missing/incomplete/invalid occurrence code(s).
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What M45 means

Occurrence codes tell the payer that something significant happened on a specific date. That might be an accident, the start of a therapy plan, the date a benefit ran out, or another event that affects coverage or payment. M45 says an occurrence code was required and missing, or the one supplied was invalid or incomplete.

Because occurrence codes often affect coordination of benefits and eligibility decisions, M45 can hold up a claim even when every service line is correct.

Common causes

  • An accident-related diagnosis was billed without the accident occurrence code and date.
  • The occurrence code was present but the date was blank or outside the claim period.
  • A code was used that does not apply to the bill type.
  • Payer-specific requirements, such as a therapy start date, were not reported.

How to fix it

  1. Identify which event the payer expected from the diagnosis, bill type, or remittance message.
  2. Add or correct the code and date in form locators 31-34.
  3. Check related fields, such as accident indicators and other insurance, for consistency.
  4. Resubmit as a new claim or adjustment according to the payer’s instructions.

How to prevent it

Tie accident-related and therapy-related registration fields to automatic occurrence code entries so they are populated before billing. Coordination issues are covered in eligibility and COB denials.

Codes that may appear with M45

  • CO-16 (Claim/service lacks information or has submission/billing error(s).): The claim is missing information or has a billing error.
  • N299 (Missing/incomplete/invalid occurrence date(s).): The occurrence date is missing or invalid, while the code itself may be fine.
  • M46 (Missing/incomplete/invalid occurrence span code(s).): An occurrence span code, which covers a date range, is the problem instead.
  • M44 (Missing/incomplete/invalid condition code.): A condition code is missing or invalid.

M45 FAQ

Where are occurrence codes on the UB-04?

In form locators 31 through 34, each with a code and a date. Electronic claims carry them in the occurrence information segment.

What events do occurrence codes report?

Examples include accident dates, onset of symptoms, dates related to benefits exhaustion, and other events defined by the National Uniform Billing Committee.

Why would an accident claim trigger M45?

If the claim indicates an accident but lacks the matching occurrence code and date, the payer cannot evaluate other liability or coordination of benefits.