The formula
(Base units + Time units + Modifying units) × Conversion factor = Payment
Base units
Each anesthesia code has a set number of base units reflecting how complex the anesthesia service is. Medicare publishes base units for each anesthesia code, and most commercial payers use similar values.
Time units
Anesthesia time starts when the anesthesia provider begins preparing the patient for anesthesia and ends when the patient is safely placed under post-anesthesia care and the provider is no longer in personal attendance. Medicare counts one time unit per 15 minutes, using the actual minutes reported. Commercial payers may round differently, so the rules in each contract matter.
Modifying units
Some payers add units for the patient's physical status (for example P3 to P5) or for qualifying circumstances such as an emergency or extreme age. Medicare does not pay these extra units, but many commercial payers do, so missing them leaves money on the table.
Conversion factor
The conversion factor is the dollar value of one unit. Medicare sets it by locality; commercial payers set it in your contract.
A worked example (illustrative)
A procedure with 5 base units, 75 minutes of anesthesia time (5 time units) and a P3 patient (1 modifying unit, where the payer recognises it) totals 11 units. At an illustrative commercial conversion factor of $60, that is 11 × $60 = $660. Record 60 minutes instead of 75, or miss the P3 unit, and the payment drops.
This article is general information, not billing, legal or tax advice. Payer rules change; always check current guidance.