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.
For the most current list of base unit values for each anesthesia procedure code can be found on the Anesthesiologist Center page on the CMS website at:
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
1-2            0.1 16-17         1.1
3               0.218             1.2
4-5            0.3 19-20        1.36              0.4 21            1.4
7-8           0.5 22-23        1.5
9              0.624            1.6
10-11       0.725-26       1.7
12            0.8 27           1.8
13-14       0.9 28-29       1.9
15           1.0 30           2.0
NOTE: Time Units are not recognized for CPT codes 01995 (Regional IV administration of local anesthetic agent or other medication (upper or lower extremity)) and 01996 (Daily hospital management of epidural or subarachnoid continuous drug administration).

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