CPT Code 01404: Anesthesia for knee disarticulation (amputation)

Key Takeaways

CPT Code 01404 describes anesthesia for open or surgical arthroscopic procedures on the knee joint that involve disarticulation (amputation) at the knee, assigned by the American Medical Association (AMA)

Anesthesia billing uses the formula (Base Units + Time Units + Modifying Units) x Conversion Factor; 01404 carries 5 base units per the ASA Relative Value Guide

01404 is often confused with 01402 (total knee arthroplasty, 7 base units) and with the 01440-01444 popliteal vascular codes; each covers a distinct procedure and none of them should be billed as 01404

Physical status modifiers P1-P6 and qualifying circumstances codes 99100, 99116, 99135, and 99140 can be reported alongside 01404 when clinically appropriate

Practice management software like Pabau helps surgical practices centralize clinical documentation and submit and reconcile insurance claims, cutting time spent chasing denied claims

CPT Code 01404 is the anesthesia code for open or surgical arthroscopic procedures on the knee joint where the procedure is a disarticulation (amputation) at the knee. It’s part of the CPT code set maintained by the American Medical Association (AMA), within the Anesthesia for Procedures on the Knee and Popliteal Area subsection.

It covers only the anesthesia component of the case: the amputation itself is billed separately under CPT 27598, the surgical code for knee disarticulation.

CPT Code 01404: Definition and clinical scope

01404 is not a general “knee and popliteal area” code. It does not cover total knee arthroplasty or vascular work in the popliteal fossa, each of which has its own code elsewhere in the 01400 series. Knowing precisely what 01404 covers, and which adjacent codes apply instead, is the difference between a clean claim and a reworked one.

Unlike evaluation and management codes billed on a flat fee, anesthesia codes like 01404 use a unit-based reimbursement formula that accounts for the complexity of the procedure, the patient’s physical condition, and the total time in service.

CPT 01404 code details at a glance

The table below summarizes the core reference data coders and billers need for CPT Code 01404. Verify base unit values against the current ASA Relative Value Guide (RVG) edition each fiscal year, as they are subject to annual review.

Procedures covered under CPT Code 01404

CPT Code 01404 applies when the anesthesia service supports a knee disarticulation: amputation through the knee joint itself, rather than through the femur or the tibial shaft. The code covers both open surgical and arthroscopic-assisted approaches to the disarticulation. Procedures that map to 01404 include:

• Knee disarticulation for peripheral vascular disease: Amputation through the knee joint when critical limb ischemia can’t be salvaged with revascularization
• Knee disarticulation for trauma: Amputation following a non-viable or mangled limb injury below the femur
• Knee disarticulation for infection: Amputation for uncontrolled osteomyelitis, gangrene, or a failed, infected prosthetic joint that can’t be salvaged with revision surgery
• Knee disarticulation for tumor resection: Amputation as oncologic treatment for a bone or soft-tissue malignancy involving the distal femur, knee joint, or proximal tibia that isn’t a candidate for limb-sparing surgery
• Revision or stump revision surgery: Further surgical management following a prior knee disarticulation, sometimes involving a plastic surgery practice for soft-tissue coverage

01404 should not be used for total knee arthroplasty, which is a joint replacement rather than an amputation and is billed as CPT 01402. It also should not be used for diagnostic or operative knee arthroscopy, such as meniscectomy, which falls under CPT 01400.

It also does not cover arterial work in the popliteal area, such as artery repair, bypass, or endarterectomy, which is billed under 01440, 01442, or 01444 depending on the specific procedure. Venous procedures in the same area, such as vein repair or ligation, fall under CPT 01430 instead, a separate code for popliteal-area veins rather than arteries.

Post-amputation rehabilitation and prosthetic fitting typically follow in a separate care episode, often coordinated with a physical therapy practice, and are not part of the anesthesia code itself.

When a procedure involves the knee and an adjacent anatomical area, coders must confirm what the operative note documents: a joint disarticulation, a joint replacement, a joint repair, or a vascular procedure each maps to a different code in the 01400-01444 range.

The same anatomical precision applies to neighboring code families: anesthesia for a procedure on the upper femur is reported under CPT 01220, not any of the knee-joint codes, even though the surgical sites sit close together.

How anesthesia billing works: The base and time formula

Anesthesia reimbursement does not follow the same flat-fee structure as most CPT codes. Instead, the CMS Physician Fee Schedule and the American Society of Anesthesiologists (ASA) use a unit-based formula that rewards appropriate complexity and time documentation. Missing any component of this formula is the most common driver of underpayment.

The formula is: (Base Units + Time Units + Modifying Units) x Conversion Factor = Reimbursement. For CPT Code 01404 billing, each component carries specific documentation requirements outlined below.

CPT 01404 base units and time units explained

Anesthesia time runs from when the anesthesia provider begins preparing the patient for anesthesia induction through to the moment they are no longer in personal attendance. Rounding conventions vary by payer: most commercial payers follow the 15-minute unit rule, but some apply 12-minute increments. Confirm payer-specific rules before submitting claims.

See how ADHD screening CPT code billing contrasts with anesthesia’s time-based model, an illustration of how different CPT families use entirely different reimbursement structures.

Physical status modifiers for CPT 01404 (P1-P6)

The ASA physical status classification system defines six patient categories, each adding modifying units to the anesthesia calculation. For CPT Code 01404 billing, selecting the correct physical status modifier is a documentation requirement, not an optional annotation.

Note that P1 and P2 add zero modifying units. Many billers omit these modifiers entirely on P1/P2 claims, which some payers flag as incomplete documentation. Include the physical status modifier on every CPT Code 01404 claim regardless of whether it adds units.

Qualifying circumstances add-on codes for CPT 01404

Qualifying circumstances codes describe extraordinary conditions that significantly affect the character and quality of the anesthesia service. They may be reported alongside CPT Code 01404 when the clinical situation warrants. Verify payer-specific bundling edits before billing, as some commercial payers have local coverage policies that restrict their use.

Code 99100 is a frequently applied qualifying circumstance in knee disarticulation billing because amputation for peripheral vascular disease and diabetic complications is disproportionately performed on patients over 70. Documentation must explicitly state the qualifying condition in the anesthesia record.

Medicare reimbursement for CPT Code 01404

Medicare reimbursement for anesthesia services follows the CMS anesthesia conversion factor (ACF), which is updated annually through the Physician Fee Schedule final rule. Knee disarticulation is a lower-volume procedure than total knee arthroplasty, but it skews toward an older, sicker Medicare population, which means claims often carry multiple qualifying circumstances and modifier combinations that draw payer scrutiny.

Use the CMS Physician Fee Schedule lookup, linked above, to find the current national ACF and locality-specific adjusters for your billing area. Refer also to procedure code fee schedules for a broader orientation to how fee schedule structures work across different payer types.

• Estimated payment range: Actual reimbursement depends on total units billed, the current ACF, and geographic adjustments; request the ACF from your MAC or verify via the CMS fee schedule tool for the current fiscal year
• MAC billing: Monitored Anesthesia Care (MAC) is not the typical anesthesia type for knee disarticulation, but where medical necessity is documented and MAC is used, the same 01404 code applies, with modifier QS appended to indicate MAC services
• CRNA supervision: Medicare requires specific modifier use for CRNAs (see CRNA billing section below); missing the correct modifier on supervision claims triggers automatic denial
• Geographic adjustment: Medicare’s Geographic Practice Cost Index (GPCI) applies to anesthesia services; practices in high-cost areas receive higher ACF-adjusted payments

Any specific dollar figure for CPT Code 01404 reimbursement must be confirmed against the current year CMS data. The ACF changes annually, making published dollar estimates stale within months of their publication date.

Streamline billing for your surgical practice

Practice management software like Pabau keeps clinical documentation and billing in one system, and Pabau's claims management software handles submission and reconciliation of your insurance claims. See how it works for your practice.

Book a demo

Documentation requirements for CPT Code 01404

CMS and commercial payers require four core documentation elements for every CPT Code 01404 claim. Incomplete records are the second most common cause of anesthesia claim denials, after incorrect code selection. For broader guidance on HIPAA compliance as it applies to anesthesia record retention, see the linked resource.

• Pre-anesthesia evaluation: Completed before the procedure; must include medical history, physical status classification (P1-P6), planned anesthetic technique, and patient risk assessment
• Intraoperative anesthesia record: Continuous record of vital signs, drug administration, fluids, anesthesia start time, and anesthesia end time; time documentation is required for accurate time unit calculation
• Post-anesthesia note: Completed before the patient is discharged from post-anesthesia care; must address respiratory function, level of consciousness, pain, and any complications
• Medical necessity narrative: Required when billing qualifying circumstances codes (99100, 99116, 99135, 99140) or when billing MAC services under Medicare; must link the clinical condition to the service provided

For guidance on standardizing medical forms across your practice, particularly for pre-anesthesia evaluations and post-anesthesia notes, see the linked Pabau resource. Standardized forms reduce omissions and make audits significantly faster.

CRNA vs. anesthesiologist billing for CPT 01404

Who performs the anesthesia service and under what supervision arrangement directly determines which modifiers accompany CPT Code 01404 on the claim. This distinction gets limited coverage in many published references, yet it’s a frequent audit target for Medicare contractors.

CRNA supervision rules vary significantly by state. Some states have opted out of the Medicare physician supervision requirement for CRNAs, allowing QZ billing without a supervising physician on-site.

Always verify the current opt-out status for your state before defaulting to a supervision modifier. Applying QX in a state where QZ applies results in a payment reduction the practice is not entitled to accept.

CPT 01404 vs. adjacent anesthesia codes in the 01400-01444 range

Selecting the correct code within the 01400-01444 family is where most CPT Code 01404 denials originate. Each code in this range covers a distinct anatomical area or procedure type. Use the comparison table and the AAPC Codify CPT lookup to confirm the correct code before submission.

The same code-family logic applies elsewhere in the anesthesia section: CPT 00404 and 01404 both start with the same first two digits despite covering entirely different anatomy, which is why the official descriptor, not the code’s neighbors, should drive selection.

The most frequent misselection in this family is billing 01402 (total knee arthroplasty, 7 base units) when the operative report actually documents a disarticulation at the knee, which is correctly reported as 01404 (5 base units), or the reverse.

Because both codes sit in the same 01400-series knee-joint block just two digits apart, a quick read of an operative note can land on the wrong one. This 2-unit swing changes reimbursement on every affected claim.

It’s also worth flagging a separate, easily-confused pair: arthroscopic ankle and foot procedures are reported with CPT 01464 (3 base units), not with 01442, which describes knee/popliteal artery endarterectomy (8 base units) and has nothing to do with the ankle or foot.

Use the FastRVU 2026 RVU lookup to calculate the monetary impact of unit differentials like these for your locality and payer mix.

Common billing errors for CPT 01404 and how to avoid them

Anesthesia billing errors cluster around a predictable set of mistakes. Catching them at the claim-building stage is far less costly than reworking a claim after a denial. The most common errors with CPT Code 01404 billing are listed below.

• Selecting 01402 instead of 01404 (or vice versa): Total knee arthroplasty (01402, 7 base units) and knee disarticulation (01404, 5 base units) describe entirely different procedures; always review the operative report before code selection
• Missing or incorrect physical status modifier: Every claim requires a P modifier; omitting it on P1/P2 cases is a common oversight that triggers payer edits
• Inaccurate time documentation: Anesthesia start and end times must appear in the intraoperative record; a missing start time makes time unit calculation impossible to verify on audit
• Unbundling qualifying circumstances: Some payers bundle 99100 into the anesthesia payment and do not allow it as a separate line; check payer-specific bundling edits before adding qualifying circumstances codes
• CRNA modifier mismatch: Using QX in a state with Medicare opt-out status (where QZ applies) reduces payment unnecessarily; verify state opt-out status annually
• Confusing 01404 with the 01440-01444 vascular codes: Popliteal artery repair, bypass, or endarterectomy are not knee disarticulation and must be billed under the correct vascular code, not 01404

How Pabau supports anesthesia and surgical billing workflows

Anesthesia billing is more documentation-intensive than most CPT billing types because every unit of reimbursement depends on verified time records, physical status notation, and modifier accuracy.

Practice management software like Pabau keeps patient records, procedure documentation, and billing in one system, so the claim reflects what was actually charted rather than a separately re-keyed log. Pabau’s claims management software then handles submission and reconciliation of the resulting insurance claims.

Practices can use Pabau’s digital forms to standardize pre-visit intake, medical history, and consent collection ahead of a procedure, creating a consistent record from the start of the patient journey. Keeping that documentation organized in one system makes it faster to pull supporting records if a payer requests them during an audit.

Getting CPT Code 01404 billing right the first time

Anesthesia billing for knee disarticulation is straightforward when the documentation is complete and the correct code is selected from the start.

The most expensive mistakes in CPT Code 01404 billing trace back to code misselection within the 01400-01444 range, particularly confusing it with total knee arthroplasty (01402) or the popliteal vascular codes (01440-01444), missing physical status modifiers, and incomplete time records, not complex regulatory ambiguity.

Practice management software like Pabau keeps clinical documentation and claims submission in one system, cutting down the back-and-forth that comes with a denied claim. To see how it works for your practice, book a demo.

Continue your research

Anesthesia codes get deleted too: CPT code 01180 was removed from the CPT set, and covers how to handle claims that still reference a retired anesthesia code.

Another code in the same numeric series: CPT code 01962 covers anesthesia for urgent hysterectomy following delivery, with its own base units and modifier rules.

Recovery doesn’t stop after surgery: HCPCS code E0185 covers the gel pressure pads used to help prevent pressure injuries during post-amputation recovery.

Comorbidities shape the anesthesia record: ICD-10 code M34.0 for progressive systemic sclerosis is the kind of systemic disease diagnosis that can push a patient’s physical status modifier higher.

Frequently Asked Questions