Anesthesia is the only specialty in the Medicare Physician Fee Schedule that bills using base units plus time units instead of relative value units. The 2026 PFS adjusted the anesthesia conversion factor and updated the ASA Crosswalk used to map surgical CPT codes to anesthesia codes. For anesthesia practices, billing accuracy is mostly about three things in 2026: capturing exact start and stop times, applying the correct supervision or medical direction modifier, and adding physical status modifiers when applicable. The anesthesia formula compounds errors quickly: a misplaced supervision modifier can convert a fully reimbursable case into a partial payment, and missing time units add up across hundreds of cases per year.
The anesthesia formula
Anesthesia reimbursement under Medicare follows a single formula: (base units + time units + modifying units) multiplied by the anesthesia conversion factor. Base units are assigned to each anesthesia CPT code (00100-01999) and reflect the relative complexity of the anesthesia required for the surgical procedure. Time units are calculated from the actual start and end of anesthesia care, with each 15 minutes counting as one unit (Medicare and most commercial payers). Modifying units come from physical status modifiers and qualifying circumstance codes when applicable. The formula is mechanical, but each input requires precise documentation.
Time units and the 15-minute increment
Anesthesia time begins when the anesthesia provider begins to prepare the patient for the induction of anesthesia in the operating room or equivalent area, and ends when the anesthesia provider is no longer in personal attendance, typically when the patient is safely placed under postoperative supervision. The most common documentation failure is rounded times that do not match the anesthesia record. Audit-ready documentation uses minute-level precision in the anesthesia record, with start and stop times that match the record exactly. Practices that round to the nearest 15 minutes lose half a unit per case on average, which compounds across volume.
Physical status modifiers (P1 through P6)
Physical status modifiers indicate the patient’s preoperative health status and add modifying units to the formula. P1 is a normal healthy patient (no additional units). P2 is a patient with mild systemic disease (no additional units under most Medicare carriers). P3 is severe systemic disease (1 additional unit). P4 is severe systemic disease that is a constant threat to life (2 additional units). P5 is a moribund patient not expected to survive without the operation (3 additional units). P6 is a brain-dead patient whose organs are being removed for donor purposes. Most commercial payers follow the Medicare structure; some adjust unit values. The documentation must support the level claimed.
Supervision and medical direction modifiers
The supervision modifiers determine which percentage of the full anesthesia fee the practice receives:
- AA: anesthesia services personally performed by the anesthesiologist (full payment).
- QK: medical direction of two, three, or four concurrent procedures by the anesthesiologist (50 percent payment to the directing anesthesiologist).
- QY: medical direction of one CRNA by an anesthesiologist (50 percent payment).
- QX: CRNA service with medical direction by a physician (50 percent payment to the CRNA).
- QZ: CRNA service without medical direction by a physician (full payment to the CRNA).
- AD: medical supervision (more than four concurrent procedures, paid at 3 base units only).
Medical direction versus medical supervision
The distinction between medical direction (QK, paid at 50 percent each) and medical supervision (AD, paid at 3 base units only) is one of the most consequential audit categories in anesthesia. Medical direction requires the anesthesiologist to perform a pre-anesthetic exam, prescribe the anesthesia plan, personally participate in the most demanding procedures, ensure that any procedures not personally performed are performed by qualified individuals, monitor the course of anesthesia at frequent intervals, remain physically present and available for diagnosis and treatment of emergencies, and provide indicated post-anesthesia care. When the anesthesiologist is supervising more than four concurrent rooms, the case shifts to medical supervision (AD). The documentation must establish that direction conditions were met for each medically directed case.
Documentation that holds up at audit
Audit-defensible anesthesia documentation includes the pre-anesthetic evaluation note, the anesthesia plan, the timed anesthesia record with minute-level start and stop, induction and emergence notes, intraoperative events and management, the post-anesthesia care unit handoff note, and (for medical direction cases) documentation of the anesthesiologist’s presence at the seven medically directing activities. The most common audit failure is medical direction billed without documentation of the seven activities. The defense is a templated medical direction note that prompts the anesthesiologist to confirm each activity for each case.
How MHB helps anesthesia practices
For anesthesia practices that want specialty-trained coders working their cases with time-unit precision, supervision-modifier discipline, and medical-direction audit support, our team supports specialty medical coding for anesthesia practices. The work runs in the practice’s existing anesthesia record system.
The bottom line
Anesthesia billing is mechanical when the documentation is precise. Time units captured to the minute, the right supervision modifier, physical status modifiers when supported, and medical direction documentation that meets the seven-activity standard. Practices that get the inputs right capture the full revenue. Practices that round times or default to AA without documentation lose meaningful revenue per case.
Authoritative sources
This article cites the following primary sources for billing-code and regulatory guidance. Always confirm current rules and codes with the publishing authority before applying to a specific claim.
