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N646 Remark Code: Assistant Payment Guidelines Applied

N646 means the payer adjusted the payment using its guidelines for services billed by an assistant, most often an assistant at surgery. Assistants are typically paid a reduced percentage of the primary surgeon's allowance, or not at all for some procedures.

Quick facts

Code
N646 (RARC N646)
Status
Active In use since July 15, 2013.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The reduction, or the full amount when the payer does not allow an assistant, is the provider's write-off under the contract or payer policy.
  • PR (Patient Responsibility): Cost-sharing on whatever portion was allowed for the assistant is reported under PR.
Official description
Reimbursement has been adjusted based on the guidelines for an assistant.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What N646 means

When a second clinician helps the primary surgeon, the payer does not usually pay that assistant the full surgical fee. It applies its assistant guidelines, which typically decide two things: whether an assistant is allowed for this procedure at all, and if so, what percentage of the primary surgeon’s allowance the assistant receives. The percentage may also differ between physician and non-physician assistants.

N646 tells you those guidelines produced the amount on the line. It can also appear when the assistant was another type of practitioner whose services a payer pays at a reduced rate.

Common causes of a larger-than-expected reduction

  • The procedure is on the payer’s list where an assistant is not payable.
  • The wrong assistant modifier was used for the type of practitioner who assisted.
  • The assistant is a non-physician practitioner paid at a lower percentage than a physician assistant at surgery.
  • The operative report does not show the assistant’s role, for procedures where documentation is required.
  • The assistant’s line was billed under the primary surgeon’s NPI, confusing the payer’s pricing.

How to fix it

  1. Check the payer’s assistant-at-surgery indicator for the procedure and the percentage it pays.
  2. Correct modifiers or provider data in box 24D and box 24J, and submit a corrected claim with resubmission code 7 if they were wrong.
  3. Request reconsideration with the operative report when the procedure allows an assistant based on medical necessity.
  4. Write off the reduction when it matches the payer’s rule.

How to prevent it

Before surgery, check whether the payer allows an assistant for the planned procedure and whether it requires documentation. Record the assistant’s name, credentials, and role in the operative note so the report supports the claim.

Codes that may appear with N646

  • CO-54 (Multiple physicians/assistants are not covered in this case.): Multiple physicians or assistants are not covered for this procedure.
  • CO-45 (Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement.): The charge exceeded the reduced allowance for an assistant.
  • N851 (Payment reduced because services were furnished by a therapy assistant.): A reduction because a therapy assistant, rather than a therapist, furnished the service.
  • N248 / N249 (Assistant surgeon name or primary identifier missing or invalid.): Data problems with the assistant's identity, rather than the payment rule.
  • N644 (Reimbursement has been made according to the bilateral procedure rule.): Another surgical pricing rule, for procedures done on both sides of the body.

N646 FAQ

Which modifiers identify an assistant at surgery?

Modifiers 80, 81, and 82 are commonly used for physician assistants at surgery, and AS is used by some payers for non-physician practitioners. Payers differ on which they accept.

Why was the assistant paid nothing?

Payers publish lists of procedures where an assistant is never allowed, allowed only with documentation, or always allowed. If the procedure is on the never-allowed list, the line is typically denied in full.

Can documentation get an assistant paid?

For procedures that allow an assistant with supporting documentation, an operative report showing why the assistant was needed may support payment on reconsideration.