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N644 Remark Code: Bilateral Procedure Payment Rule

N644 means the payer priced the service using its bilateral procedure rule, which sets how much it pays when the same procedure is done on both sides of the body. The allowed amount is usually more than one side but less than double.

Quick facts

Code
N644 (RARC N644)
Status
Active In use since July 15, 2013.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The reduction below the full charge for both sides is a contractual adjustment and is not billed to the patient.
  • PR (Patient Responsibility): Deductible or coinsurance on the bilateral allowed amount is reported separately under PR.
Official description
Reimbursement has been made according to the bilateral procedure rule.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What N644 means

A bilateral procedure is the same service performed on matching left and right body parts in the same session. Most payers do not pay two full fees for it. Under a typical bilateral rule, the payer pays a set percentage of the single-side amount, often more than one side and less than double, and the remittance shows N644 to tell you that rule shaped the payment.

Payers assign each procedure a bilateral indicator that controls the math. Some codes allow a bilateral adjustment, some are already described as bilateral and pay once, and some cannot be billed bilaterally at all. The rule is often combined with multiple procedure pricing, which is why N644 frequently rides with CARC 59.

When to look closer

  • Paid at one side only when both sides were done and the code allows bilateral payment.
  • Denied a line because the claim used RT and LT on separate lines when the payer wanted modifier 50, or the other way round.
  • Units wrong. Modifier 50 with two units can cause a payer to double-count or reject the line.
  • Bilateral and multiple procedure rules stacked in an order that does not match the payer’s policy.

How to fix it

  1. Look up the payer’s bilateral indicator for the procedure and its required billing format.
  2. Recalculate the expected payment with the payer’s percentage and compare it to the allowed amount.
  3. If the format was wrong, submit a corrected claim with resubmission code 7 in box 22, adjusting the modifiers in box 24D and units in box 24G.
  4. If the math is wrong, contact provider services or request reconsideration with the calculation.

How to prevent it

Keep a payer-by-payer note on bilateral billing format and build it into charge entry so surgical and imaging lines are coded consistently. For modifier-driven edits generally, see NCCI edits and modifiers.

Codes that may appear with N644

  • CO-59 (Processed based on multiple or concurrent procedure rules. (For example multiple surgery or diagnostic imaging, concurrent anesthesia.)): Processed under multiple or concurrent procedure rules, which include bilateral pricing.
  • CO-45 (Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement.): Charge exceeded the allowed amount after the bilateral adjustment.
  • N670 (This service code has been identified as the primary procedure code subject to the Medicare Multiple Procedure Payment Reduction (MPPR) rule.): The multiple procedure payment reduction, which applies to different procedures rather than the same one on both sides.
  • CO-4 (The procedure code is inconsistent with the modifier used.): The procedure code is inconsistent with the modifier, such as modifier 50 on a code that cannot be bilateral.
  • N646 (Reimbursement has been adjusted based on the guidelines for an assistant.): Payment adjusted under the payer's assistant guidelines, another rule-based surgical reduction.

N644 FAQ

Should I bill bilateral procedures with modifier 50 or with RT and LT?

It depends on the payer. Some want one line with modifier 50 and one unit, others want two lines with RT and LT. Using the wrong format can cause either an underpayment or a denial.

Why was I paid for only one side?

The payer may treat the code as inherently bilateral, meaning the description already covers both sides, or it may not have recognized the bilateral modifier. Check the payer's indicator for that code.

Is N644 a denial?

No. It explains how the payment was calculated. Review it only if the amount does not match the payer's published bilateral rule.