Anesthesia Coding Basics: Base Units, Time, and Modifiers

Code sets: CY 2026 CPT and HCPCS Level II.

Anesthesia coding uses the CPT anesthesia section (00100-01999), but payment works differently from other services. It combines base units, time, and modifiers.

How the code is chosen

Anesthesia codes are organized by body area and procedure type. Coders crosswalk the surgical CPT code to the matching anesthesia code. When more than one procedure is performed, report the anesthesia code with the highest base unit value.

How payment is calculated

(Base units + time units) x anesthesia conversion factor, adjusted by the modifiers.

• Base units reflect the complexity of the anesthesia service and come from the ASA Relative Value Guide (Medicare publishes its own base units).
• Time starts when the anesthesia practitioner begins preparing the patient for anesthesia in the operating room or equivalent area, and ends when the practitioner is no longer in personal attendance. Medicare requires actual anesthesia time in minutes on the claim; it is converted to units (typically 15 minutes per unit) (Medicare Claims Processing Manual, Chapter 12, Section 50).

Payment modifiers

• AA: anesthesia performed personally by the anesthesiologist.
• QK: medical direction of two, three, or four concurrent procedures by the anesthesiologist.
• QX: CRNA service with medical direction by a physician.
• QY: medical direction of one CRNA by an anesthesiologist.
• QZ: CRNA service without medical direction.
• QS: monitored anesthesia care (informational, used with the payment modifier).

Physical status and qualifying circumstances

Physical status modifiers P1-P6 describe the patient’s health, and qualifying circumstance add-on codes (99100-99140) describe factors like extreme age or emergency conditions. Many commercial payers add units for these; Medicare does not pay separately for them. Check each payer.

Documentation to look for

• Anesthesia start and stop times.
• Type of anesthesia (general, regional, MAC).
• Who performed or directed the service, and how many concurrent cases.
• The surgical procedure, to support the crosswalk.

Refreshed from the CCO Community archive, November 2026.