Time Units, Qualifying Circumstances & CRNA Rules
Anesthesia billing operates on a unit-based formula unique in all of medicine, and the formula is deceptively simple on paper but full of compliance traps in practice. Getting base units wrong, mis-documenting time, or applying CRNA modifiers incorrectly can turn a profitable anesthesia program into a denial and audit magnet.
If you're working through these issues, our specialty billing services can help you address them systematically.
Unlike other specialties that bill by CPT code alone, anesthesia reimbursement is calculated as:
(Base Units + Time Units + Qualifying Circumstance Units) × Conversion Factor = Allowed Amount
Each element of this formula has specific rules. Understanding all three, and how payers verify them, is the foundation of compliant anesthesia billing.
Anesthesia base units reflect how difficult and risky a procedure is. CMS assigns them by CPT code, and they didn't change for 2026. For example, 00400 is 3 units, 00700 is 4, 01400 is 4, 00840 is 6 and 00600 is 10. Time units are added on top of the base units.
Base units reflect the complexity and risk of each anesthesia procedure. They're assigned by the American Society of Anesthesiologists (ASA) and published in the ASA Relative Value Guide, and CMS adopts these values with occasional modifications. CMS says the base units are unchanged for 2026.
Base units are determined by: the complexity of the surgical procedure, the positioning of the patient, the monitoring required, and the likelihood of complications. They don't change based on how long the case takes; that's what time units cover.
| CPT | Procedure | Base units |
|---|---|---|
| 00100 | Salivary glands, mouth | 5 |
| 00300 | Head, neck, trunk skin and muscle | 5 |
| 00400 | Integumentary system, extremities, anterior trunk | 3 |
| 00600 | Cervical spine and cord | 10 |
| 00700 | Upper anterior abdominal wall | 4 |
| 00731 | Upper GI endoscopy | 5 |
| 00732 | Upper GI endoscopy with ERCP | 6 |
| 00790 | Intraperitoneal, upper abdomen | 7 |
| 00811 | Lower intestinal endoscopy, not screening | 4 |
| 00812 | Screening colonoscopy | 3 |
| 00840 | Intraperitoneal, lower abdomen | 6 |
| 00944 | Vaginal hysterectomy | 6 |
| 00952 | Hysteroscopy and related vaginal procedures | 4 |
| 01400 | Knee joint, open or arthroscopic | 4 |
| 01402 | Total knee arthroplasty | 7 |
| 01610 | Shoulder and axilla | 5 |
| 01830 | Wrist and hand, arthroscopy | 3 |
| 01961 | Cesarean delivery | 7 |
| 01967 | Neuraxial labor analgesia | 5 |
These values match the CMS anesthesia base unit file. Check that file for any code not listed here. The old lower intestinal endoscopy code 00810 was deleted and replaced by 00811 and 00812.
Anesthesia payment equals base units plus time units, multiplied by the conversion factor. Medicare counts one time unit per 15 minutes, so a 90-minute case adds 6 time units. With 4 base units, total units of 10 at the 2026 national conversion factor of $20.4976 pay about $205 before locality adjustment.
Try a real code. A total knee arthroplasty (01402) has 7 base units. A 90-minute case adds 6 time units for 13 total units, and 13 x $20.4976 is $266.47 at the national factor. The locality factor moves that number: the CMS file lists $19.53 for Alabama and $28.15 for Alaska (non-APM), which makes the same case $253.89 in Alabama.
Time units are calculated at 1 unit per 15 minutes of anesthesia time under Medicare and most commercial payers. Anesthesia time begins when the anesthesiologist (or CRNA) starts preparing the patient for induction and ends when the patient is safely transferred to post-anesthesia care.
Important: Anesthesia time is NOT the same as surgical time. It includes pre-induction preparation, intraoperative monitoring, and handoff to PACU. Always document start and end times explicitly in the anesthesia record, payers audit this on medical necessity reviews.
Payers handle partial 15-minute units differently. Medicare takes the total minutes you report and divides by 15 (17 minutes is 1.13 units); it doesn't round to whole units. Many commercial payers round up to the next full unit. Bill per your payer contracts, applying the wrong rounding method is a common source of underpayment.
Example: A 47-minute case = 47 ÷ 15 = 3.13 time units under Medicare. Under many commercial plans: 4 units.
Qualifying circumstances are add-on codes that add units when anesthesia is provided under especially challenging conditions. Medicare's fee schedule lists 99100, 99116, 99135 and 99140 as bundled (status B), so Medicare adds no payment for them. Many commercial payers do recognize them, and practices often under-bill them there.
| Code | Qualifying Circumstance | Additional Units |
|---|---|---|
| 99100 | Anesthesia for patient under 1 year or over 70 years | +1 unit |
| 99116 | Anesthesia complicated by use of controlled hypotension | +5 units |
| 99135 | Anesthesia complicated by use of induced hypothermia | +5 units |
| 99140 | Anesthesia complicated by emergency conditions | +2 units |
99100 is frequently missed: Any patient under age 1 or over age 70 qualifies, this applies to a large portion of surgical patients in most practices. With a commercial payer that recognizes 99100 at a $70 conversion factor, one missed unit is $70 per case. If you're not appending 99100 to eligible commercial cases, audit your last 90 days.
The conversion factor translates total units into dollars. Medicare sets a national base conversion factor, then adjusts by geographic locality (GPCI). In 2026, the Medicare national anesthesia conversion factor is $20.4976 per unit ($20.5998 for clinicians in a qualifying Advanced APM), modified by the anesthesia locality factor for your area.
Commercial payers set their own conversion factors through contract negotiation. Know your contracted conversion factor by payer and verify claims are processing at the correct rate quarterly.
Certified Registered Nurse Anesthetists (CRNAs) can bill independently or under physician supervision, and the modifier determines payment rate and compliance requirements.
| Modifier | Meaning | Medicare Payment |
|---|---|---|
| QZ | CRNA without physician medical direction | 100% of allowed amount |
| QX | CRNA with physician medical direction | 50% to CRNA |
| QK | Medical direction of 2–4 CRNAs by physician | 50% to physician |
| QY | Medical direction of 1 CRNA by anesthesiologist | 50% to physician |
| AA | Anesthesia services performed personally by anesthesiologist | 100% of allowed amount |
| AD | Medical supervision of more than 4 concurrent procedures | 3 base units to physician |
When billing medical direction, the supervising anesthesiologist must fulfill all 7 CMS requirements for each case:
Audit risk: OIG has repeatedly flagged medical direction billing where anesthesiologists were directing more than 4 concurrent cases or where documentation of the 7 requirements was absent or incomplete. Missing documentation of even one element creates False Claims Act exposure.
Monitored Anesthesia Care (MAC) is performed when an anesthesiologist or CRNA provides sedation and monitoring without general anesthesia. MAC is billed using the same anesthesia CPT codes with modifier QS (monitored anesthesia care service).
Medicare pays MAC on the same basis as other anesthesia when it's reasonable and medically necessary, and contractors publish their own policies and diagnosis requirements for endoscopy anesthesia. Billing MAC for a routine colonoscopy without a documented reason is a common review target, so check your contractor's current policy.
Time-unit and modifier errors are the two things that quietly erode anesthesia collections month over month. A compliance audit of your last quarter's anesthesia claims will show you exactly where base units, time conversion, or CRNA modifiers are costing you, start with a free revenue assessment.
See how Rcmaxis handles the full scope of Anesthesia billing on our Anesthesia billing services page.
Anesthesia claims turn on documentation that has to agree with itself. The start and stop times on the anesthesia record must match the times on the claim, and the modifier has to match who actually did what. When a claim says medically directed but the record doesn't show the physician's required involvement at the key points, the payment level is wrong, and reviewers know exactly where to look.
The OIG has published work on anesthesia medical direction billing, and contractors use the same logic on their own reviews. The lookback is typically three years, and a sample-based finding can be extrapolated across every anesthesia claim in that period. A 3-year lookback with a recurring modifier-mismatch pattern can put a large recoupment at stake, before the appeal costs. A quarterly check that compares 20 or 30 anesthesia records to their claims, looking only at times and modifiers, catches most of the problems before an auditor does.
Anesthesia payment equals base units plus time units, multiplied by the conversion factor. Medicare counts one time unit per 15 minutes, so a 90-minute case adds 6 time units. With 4 base units, total units of 10 at the 2026 national conversion factor of $20.4976 pay about $205 before locality adjustment.
Anesthesia base units reflect how difficult and risky a procedure is. CMS assigns them by CPT code, and they didn't change for 2026. For example, 00400 is 3 units, 00700 is 4, 01400 is 4, 00840 is 6 and 00600 is 10. Time units are added on top of the base units.
No. The 2026 Medicare fee schedule file lists 99100, 99116, 99135 and 99140 as bundled (status B), so they add no payment. Some commercial payers do recognize them as extra units, so check each contract before you bill them.
Anesthesia payment uses a formula: base units plus time units plus any qualifying circumstance units, multiplied by the conversion factor. Time is counted at one unit per 15 minutes under Medicare and most commercial payers, so accurate start and stop times matter to the dollar.
The modifier states who provided the service and under what supervision, and it sets the payment rate. QZ is a CRNA working without physician supervision, and the article lists the others. Use the one your record supports, because the payer will check it against the documentation.
MAC uses the same anesthesia CPT codes with modifier QS. It applies when an anesthesiologist or CRNA provides sedation and monitoring without general anesthesia, so the record should show the monitoring and the patient's status throughout.
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