what is base unit and time unit.

Base Units

Each anesthesia code (procedure codes 00100-01999) is assigned a base unit value by the
American Society of Anesthesiologists (ASA) and used for the purpose of establishing fee
schedule allowances.

Anesthesia services are paid on the basis of a relative value system, which include both base and
actual time units. Base units take into account the complexity, risk, and skill required to perform
the service.

Time Units
Anesthesia time is defined as the period during which an anesthesia practitioner is present with
the patient. It starts when the anesthesia practitioner begins to prepare the patient for anesthesia
services in the operating room or an equivalent area and ends when the anesthesia practitioner is
no longer furnishing anesthesia services to the patient, that is, when the patient may be placed
safely under postoperative care.

Anesthesia time is a continuous time period from the start of anesthesia to the end of an
anesthesia service. In counting anesthesia time for services furnished, the practitioner can add
blocks of time around an interruption in anesthesia time as long as the anesthesia practitioner is
furnishing continuous anesthesia care within the time periods around the interruption.

For anesthesia claims, the elapsed time, in minutes, must be reported. Convert hours to minutes
and enter the total minutes required for the procedure in Item 24G of the CMS-1500 claim form or
electronic media claim equivalent.
Time units for physician and CRNA services – both personally performed and medically directed
are determined by dividing the actual anesthesia time by 15 minutes or fraction thereof. Since
only the actual time of a fractional unit is recognized, the time unit is rounded to one decimal
place. The table below illustrates the conversion from minutes to units used by the carrier for

Minutes Units
Minutes Units
1-2 0.1 16-17 1.1
3 0.2 18 1.2
4-5 0.3 19-20 1.3
6 0.4 21 1.4
7-8 0.5 22-23 1.5
9 0.6 24 1.6
10-11 0.7 25-26 1.7
12 0.8 27 1.8
13-14 0.9 28-29 1.9
15 1.0 30 2.0

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