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N203 Remark Code: Anesthesia Time or Units Invalid

N203 means the anesthesia time or units on the claim were missing, incomplete, or invalid. Anesthesia payment depends on time, so the payer could not calculate the allowance without correct minutes or units.

Code: N203 (RARC N203)
Status: Active In use since June 30, 2003; last modified March 14, 2014.
Code set: Remittance Advice Remark Codes (RARC)
Group codes: CO (Contractual Obligation): The anesthesia line was denied or reduced because time or units could not be accepted. Correct it; the patient is not responsible for the error.
Official description: Missing/incomplete/invalid anesthesia time/units.
X12 Remittance Advice Remark Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What N203 means

Anesthesia services are paid differently from most professional services. The allowance is built from base units for the procedure plus time, and sometimes modifying units. If the time or units field is blank, zero, formatted in an unexpected way, or inconsistent with the payer’s rules, the payer cannot price the service and returns N203, usually alongside CARC 16.

Common causes

Units were entered as time units when the payer expected total minutes, or vice versa.
Base units were added to the units field even though the payer adds them itself.
Time was rounded differently from the payer’s rules.
Start and stop times were required but missing, or they did not match the reported total.
A line for an anesthesia service was billed with a units value of 1 by default.

How to fix it

Pull the anesthesia record and calculate total anesthesia time from start to end.
Look up the payer’s instructions for reporting anesthesia: minutes versus units, whether to include base units, and where times go.
Correct box 24G (and any time fields the payer requires, such as box 19) to match those instructions.
Check that the anesthesia modifiers, such as AA, QK, QX, QY, or QZ, match who performed the service and the supervision ratio.
Send a corrected claim with resubmission code 7 and the original claim number.

How to prevent it

Set payer-specific anesthesia rules in your billing system so time is converted automatically. Audit a sample of anesthesia claims against records periodically, particularly after contracts change. For a broader look at units problems, see MUE and units of service denials.

Codes that may appear with N203

CO-16 (Claim/service lacks information or has submission/billing error(s).): The claim lacks information needed to adjudicate; N203 pinpoints anesthesia time or units.
CO-151 (Payment adjusted because the payer deems the information submitted does not support this many/frequency of services.): The payer deems the information does not support the number of units or frequency billed.
CO-59 (Processed based on multiple or concurrent procedure rules. (For example multiple surgery or diagnostic imaging, concurrent anesthesia.)): Processed under concurrent anesthesia or multiple procedure rules, which interacts with reported time.

N633 (Additional anesthesia time units are not allowed.): Additional anesthesia time units are not allowed.
N635 (The Allowance is calculated based on the anesthesia base units plus time.): The allowance was calculated using anesthesia base units plus time.
N439 (Missing anesthesia physical status report/indicators.): Missing anesthesia physical status report or indicators.

N203 FAQ

Should anesthesia be billed in minutes or units?

It depends on the payer. Medicare and many others want total minutes in the units field, while some commercial and Medicaid plans want time units. Follow each payer's instructions.

Where do start and stop times go?

Many payers only need total time, but some ask for start and stop times, often in box 19 or an equivalent note field. Check the payer's anesthesia billing guide.

What if anesthesia time spans midnight?

Report total continuous time accurately. Some payers have specific instructions for services that cross into another date, so confirm before submitting.

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