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N635 Remark Code: Anesthesia Base Units Plus Time

N635 means the payer calculated the anesthesia allowance using the base units for the procedure plus time units. This is the standard anesthesia formula: base units and time units are added, then multiplied by the payer's conversion factor.

Quick facts

Code
N635 (RARC N635)
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 base-plus-time allowance is a provider adjustment.
Official description
The Allowance is calculated based on the anesthesia base units plus time.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What N635 means

Most anesthesia payment follows one formula: (base units + time units) × conversion factor. Base units reflect how complex the anesthesia service is, and time units reflect how long it lasted. N635 tells you the payer used that formula for this line.

It is usually paired with CARC 45 or P12.

Why the allowance may differ from your expectation

  • Different base units. The payer may use a different base-unit source or edition, or crosswalk the surgical procedure to a different anesthesia code.
  • Fewer time units, because of rounding rules or documented start and stop times.
  • No modifying units, where your contract or fee schedule doesn’t recognize them.
  • Reductions for medical direction or concurrency, reported with anesthesia modifiers and sometimes CARC 59.
  • A different conversion factor than the one in your contract.

How to check it

  1. Identify the anesthesia code and base units the payer used.
  2. Confirm time units from your documented minutes and the payer’s rules.
  3. Apply the conversion factor and any modifier reductions, then compare.
  4. Correct or dispute. Send a corrected claim with resubmission code 7 for reporting errors, or request repricing if the payer’s inputs were wrong.

Staying accurate

Keep the base-unit source, time rules, and conversion factor for each payer in your system, and document start and end times precisely. For unit-related edits more broadly, see units of service denials.

Codes that may appear with N635

  • 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.
  • N634 (The allowance is calculated based on anesthesia time units.): The allowance is calculated on time units alone.
  • N633 (Additional anesthesia time units are not allowed.): Additional anesthesia time units were not allowed.
  • 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, including concurrent anesthesia.

N635 FAQ

Where do base units come from?

From a relative value guide that assigns base units to each anesthesia service. Payers and fee schedules specify which guide and edition they use.

Are physical status or other modifying units included?

Some payers add units for physical status or qualifying circumstances; others, including Medicare, don't pay them. N635 mentions only base and time, so ask the payer if you expected more.

Is N635 a denial?

No. It describes how the allowance was calculated.