Short answerAnesthesia payment is generally (base units + time units + any modifying units) multiplied by the payer's conversion factor. Time runs from when the anesthesia provider starts preparing the patient to when the patient is safely placed under post-anesthesia care, and is commonly converted at 15 minutes per unit; Medicare requires actual minutes on the claim.
Anesthesia is billed differently from almost every other specialty. Small differences in time or modifiers repeat on every case, so they add up.
The formula
Payment = (base units + time units + modifying units) × conversion factor
- Base units: fixed per anesthesia code, reflecting procedure complexity.
- Time units: anesthesia time divided into units, commonly 15 minutes each.
- Modifying units: some payers add units for physical status (P3–P5) or qualifying circumstances.
A worked example
| Item | Value |
|---|---|
| Base units | 5 |
| Anesthesia time | 72 minutes = 4.8 units at 15 minutes per unit |
| Total units | 9.8 |
| Conversion factor (example) | $60.00 |
| Expected allowed | $588.00 |
If a payer rounds time down to 4 units, the allowed amount falls to $540: a $48 loss on one case.
Medical direction modifiers
| Modifier | Meaning | Medicare payment effect |
|---|---|---|
| AA | Personally performed by the anesthesiologist | Full fee |
| QK | Medical direction of 2–4 concurrent cases | 50% to the anesthesiologist |
| QX | CRNA service with medical direction | 50% to the CRNA |
| QY | Medical direction of one CRNA | 50% to the anesthesiologist |
| QZ | CRNA without medical direction | Full fee to the CRNA |
Where groups lose revenue
- time units rounded or capped against the contract
- start and stop times not documented clearly
- medical direction pairs that don't match (a QK line with no matching QX line)
- physical status modifiers missing where the contract pays them
Related: anesthesia billing and anesthesia and the No Surprises Act.
This article is general billing information, not legal or coding-compliance advice. Payer rules, CPT descriptors and federal guidance change; check the current versions for your claims.