Anesthesiology billing operates on a unit-based system that differs from every other medical specialty. Rather than a flat fee per procedure, CMS reimburses anesthesia services by combining base units assigned to each procedure code with time units calculated in 15-minute increments. Getting this calculation wrong by even one unit can trigger a claim denial or underpayment, making code-level accuracy critical for every anesthesiology practice.
The Centers for Medicare and Medicaid Services (CMS) publishes annual conversion factors for anesthesia reimbursement. The 2026 anesthesia conversion factor is $20.23 per unit under the Medicare Physician Fee Schedule (MPFS). Each anesthesia CPT code carries a fixed base unit value, and the total reimbursement equals (base units + time units + qualifying circumstance units) multiplied by the conversion factor and adjusted by geographic locality.
Core Anesthesiology CPT Codes and Base Unit Values
The American Medical Association (AMA) CPT code set assigns anesthesia codes in the 00100-01999 range. Each code represents anesthesia services for a specific anatomical region or procedure type. The base unit value reflects the relative complexity and risk involved in providing anesthesia for that procedure.
CPT 00100 covers anesthesia for procedures on the salivary glands, including biopsy, with a base unit value of 5. This is one of the lower-complexity codes and is billed when an anesthesiologist or CRNA administers anesthesia for head and neck procedures in the oral cavity region.
CPT 00400 covers anesthesia for procedures on the integumentary system of the extremities, anterior trunk, and perineum. With a base unit value of 3, this code applies to skin grafts, excisions, and wound repairs on the arms, legs, and torso. The lower base unit reflects the generally lower physiological risk compared to thoracic or abdominal procedures.
CPT 00740 covers anesthesia for upper gastrointestinal endoscopic procedures, including esophagoscopy and EGD. The base unit value is 7. This code is frequently billed in ambulatory surgical centers and hospital outpatient departments where GI procedures are performed under anesthesia rather than moderate sedation.
CPT 00810 covers anesthesia for lower intestinal endoscopic procedures, including colonoscopy. Base unit value is 7. As colonoscopy volumes remain high across outpatient settings, this is one of the most frequently billed anesthesia codes. Payer policies vary significantly on whether propofol administration by an anesthesiologist is medically necessary for routine colonoscopy.
CPT 01996 covers daily hospital management of epidural or subarachnoid continuous drug administration. Base unit value is 3. This code is billed each day the anesthesiologist manages a continuous epidural, typically for post-surgical pain control or labor analgesia. It is not time-based and is billed once per day regardless of visit length.
CPT 99100 is a qualifying circumstance add-on code representing anesthesia for patients of extreme age, specifically newborns through age 1 year and patients 70 years and older. It adds 1 qualifying unit to the base and time unit total. It is reported in addition to the primary anesthesia code, not as a standalone charge.
Time Unit Calculation and Documentation Requirements
Time units are calculated by dividing total anesthesia time by 15 minutes. CMS rounds to the nearest whole unit. Anesthesia time begins when the anesthesiologist starts preparing the patient in the operating room or equivalent area and ends when the anesthesiologist is no longer in personal attendance. The start and stop times must appear on the anesthesia record, which serves as the primary billing documentation.
For a 45-minute procedure with CPT 00740 (7 base units), the calculation is: 7 base units + 3 time units = 10 total units. At the 2026 conversion factor of $20.23 per unit, the Medicare allowable is $202.30 before geographic adjustment. Commercial payers use their own conversion factors, which may range from $22 to $35 per unit depending on the payer and region.
Modifier Usage for Anesthesiology Claims
Anesthesia claims require a specific set of modifiers that communicate the care delivery model to payers. Modifier AA indicates the anesthesiologist personally performed the anesthesia service. Modifier QK indicates the anesthesiologist medically directed two to four concurrent procedures by CRNAs. Modifier QX is appended to the CRNA claim when the CRNA works under medical direction by an anesthesiologist. Modifier QY indicates the anesthesiologist medically directed one CRNA. Modifier QZ indicates the CRNA performed the service without medical direction.
Medical direction rules under the Anesthesia Care Team (ACT) model require the anesthesiologist to perform seven specific functions: pre-anesthesia evaluation, prescription of the anesthesia plan, personal participation in the most demanding portions of the procedure, monitoring of the case, remaining physically available for immediate diagnosis and treatment, providing indicated post-anesthesia care, and ensuring the CRNA is competent. Failure to document all seven functions can result in a downgrade from the medical direction rate (50% of the physician rate for each concurrent procedure) to the medical supervision rate.
Common Billing Errors in Anesthesiology
The most frequent anesthesiology billing error is incorrect time unit calculation, typically caused by rounding errors or failure to document start and stop times precisely. The second most common error is modifier mismatch, where the attending anesthesiologist bills AA (personally performed) while the CRNA on the same case bills QX (medical direction), creating a contradiction that triggers an automatic denial or audit flag. The third common error is billing CPT 01996 for epidural catheter placement rather than daily management, confusing the placement code (which is included in the surgical procedure’s anesthesia allowance) with the ongoing management code.