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N636 Remark Code: Payable Only Once Per Injury

N636 means the payer adjusted the service because its rules allow it to be paid only once for a given injury. It shows up mainly on workers' compensation and property and casualty claims where an earlier bill for the same injury already used that single allowance.

Quick facts

Code
N636 (RARC N636)
Status
Active In use since July 15, 2013.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The payer treats the repeat charge as the provider's adjustment under the one-per-injury rule. Whether the injured person can be billed depends on the jurisdiction's rules, and many workers' compensation systems prohibit it.
  • PR (Patient Responsibility): Less common. The payer is indicating the patient may owe the amount. Confirm this is allowed under the applicable workers' compensation or auto insurance rules before billing anyone.
Official description
Adjusted because this is reimbursable only once per injury.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What N636 means

Some services are designed to happen a single time for each injury: an initial evaluation, a baseline report, certain one-time supplies, or a specific type of assessment. When a carrier’s fee schedule or policy says a service is payable only once per injury and it sees that service billed again under the same claim, it adjusts the new line and attaches N636 to explain why.

This remark is at home on workers’ compensation and property and casualty bills (auto personal injury protection, liability, and similar coverage). The “injury” is identified by the carrier’s claim number and the date of injury or date of loss. That is why the remark is usually tied to a reason code such as CARC 119 (benefit maximum reached for the occurrence).

Common causes

  • The same one-time service was already paid earlier in the case, possibly to a different provider in the same practice or network.
  • A second injury was billed under the first injury’s claim number, so the carrier matched it to the earlier allowance.
  • The service was coded as an initial or one-time service when it was really a follow-up that should have been reported differently.
  • The carrier’s records merged two claims, or the earlier payment was later reversed but the limit was still applied.

What to do

  1. Pull the claim history for this injury. Look for the earlier payment of the same service and note who billed it and when.
  2. Confirm the claim number and date of injury on your bill match the injury you actually treated. If they belong to a different injury, send a corrected bill under the right claim.
  3. Check the coding. If a follow-up service was reported as an initial one, correct it and resubmit according to the carrier’s corrected-bill rules.
  4. Dispute if the limit was misapplied. If the earlier service was never paid, or was reversed, request reconsideration with the payment history attached.
  5. Know the jurisdiction’s rules before billing the injured person. Many state workers’ compensation systems restrict balance billing, and the rules differ by state.

How to prevent it

Keep a per-claim log of one-time services for every workers’ compensation and auto case, and check it before scheduling another. When a patient reports a new injury, open a separate case with its own claim number and date of injury rather than reusing the old one.

Codes that may appear with N636

  • PR-119 (Benefit maximum for this time period or occurrence has been reached.): Benefit maximum for the period or occurrence has been reached, which here means the one allowance tied to the injury.
  • CO-219 (Based on extent of injury.): Adjustment based on extent of injury, sometimes used when the carrier ties allowances to the accepted injury.
  • N640 (Exceeds number/frequency approved/allowed within time period.): A frequency limit measured over a time period rather than over the life of an injury claim.
  • N666 (Only one evaluation and management code at this service level is covered during the course of care.): Limits one evaluation and management service at a given level during the course of care.
  • N653 (The date of injury does not match the reported date of loss.): The date of injury on the bill does not match the loss date the carrier has on file.
  • OA-18 (Exact duplicate claim/service (Use only with Group Code OA except where state workers' compensation regulations requires CO)): An exact duplicate claim, which is a different problem from a legitimately separate service that is limited per injury.

N636 FAQ

Is N636 used by group health plans?

It is mostly seen on workers' compensation and property and casualty remittances, where benefits are organized around a specific injury or date of loss. A group health plan would usually express a similar limit as a frequency or benefit maximum.

What if the patient has a second, separate injury?

Then the service may be payable again, but the bill has to be filed under the claim number and date of injury for that second injury. If it was filed under the first claim, the carrier will see it as a repeat.

Can I appeal an N636 adjustment?

Yes, if you believe the rule was misapplied, for example because the earlier service was never paid or belonged to a different injury. Follow the carrier's reconsideration process and the jurisdiction's dispute rules.