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N634 Remark Code: Allowance Based on Anesthesia Time

N634 means the payer calculated the allowance based on anesthesia time units. It explains the pricing method, so the amount paid reflects the recognized time units multiplied by the payer's conversion factor, possibly with other adjustments.

Quick facts

Code
N634 (RARC N634)
Status
Active In use since July 15, 2013.
Code set
Remittance Advice Remark Codes (RARC)
Group codes
  • CO (Contractual Obligation): The difference between the charge and the time-based allowance is a provider adjustment.
Official description
The allowance is calculated based on anesthesia time units.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What N634 means

N634 is a pricing explanation for anesthesia services. It tells you the payer arrived at the allowance using time units, the length of anesthesia converted into units under the payer’s rules, multiplied by a conversion factor.

It commonly appears with CARC 45 or P12.

How to check the payment

  1. Confirm the time the payer recognized. Divide your documented minutes using the payer’s unit rule, often 15-minute units, though rules vary.
  2. Find the conversion factor in the payer’s fee schedule or contract for the date of service.
  3. Multiply and compare with the allowance.
  4. Look for missing base units. If the payer’s method should include base units and the remittance shows only time, ask why. N635 indicates a base-plus-time calculation.
  5. Check modifiers, since some anesthesia modifiers reduce payment, such as medical direction of residents or CRNAs.

If the amount is wrong

Request repricing with your calculation, or submit a corrected claim with resubmission code 7 if the reported time, units, or modifiers were wrong.

Staying accurate

Load each payer’s anesthesia conversion factor and time rules into your billing system so allowances can be checked when payments post. An ERA analysis can flag anesthesia payments that don’t match the expected formula.

Codes that may appear with N634

  • CO-45 (Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement.): The charge exceeds the fee schedule or maximum allowable amount.
  • CO-P12 (Workers' compensation jurisdictional fee schedule adjustment.): A workers' compensation jurisdictional fee schedule adjustment.
  • N635 (The Allowance is calculated based on the anesthesia base units plus time.): The allowance is calculated on base units plus time, rather than time alone.
  • N633 (Additional anesthesia time units are not allowed.): Additional anesthesia time units were not allowed.
  • N203 (Missing/incomplete/invalid anesthesia time/units.): Anesthesia time or units are missing, incomplete, or invalid.

N634 FAQ

Why weren't base units included?

N634 says the allowance was calculated on time units. If you expected base units as well, ask the payer whether its fee schedule prices this service on time alone or whether base units were missed.

Is N634 a denial?

No. It explains how the allowed amount was calculated.

What conversion factor did the payer use?

It depends on the payer and fee schedule. Workers' comp schedules and contracts set their own anesthesia conversion factors.