Modifiers · HCPCS modifier

PA / NP / CNS assistant at surgery

Modifier AS identifies when a physician assistant, nurse practitioner, or clinical nurse specialist steps in as the assistant surgeon during an operative case. It tells Medicare and other payers that a non-physician mid-level provider—not a physician or resident—filled the assistant-at-surgery role. Reimbursement is typically set at 85% of the assistant-at-surgery allowable, which itself is 16% of the primary surgeon's fee schedule amount.

Verified May 8, 2026 · 7 sources ↓

Type: HCPCS
CPT codes use it: 527
Top regions: Spine, Other, Foot & ankle

Drawn from CMSAAPCNovitas SolutionsPalmetto GBAAMA

When to use modifier AS

Source · Editorial brief grounded in 7 cited references ↓

Append modifier AS to the primary surgery CPT code on the PA's, NP's, or CNS's own claim when that provider physically assists the operating surgeon throughout the procedure. The modifier belongs on the assistant's claim line—not on the primary surgeon's claim. For example, if a PA assists during a total knee arthroplasty (CPT 27447), the PA's practice bills 27447-AS; the orthopedic surgeon bills 27447 without any assistant modifier. Medicare requires the procedure to be designated as 'payment allowed for assistant at surgery' in the Medicare Physician Fee Schedule (indicator 1 or 2) before the claim will pay.

Do not stack modifier AS with CPT modifiers 80, 81, or 82. Those three modifiers are reserved exclusively for physician assistants at surgery. When a PA, NP, or CNS is the assistant, modifier AS is the only assistant-at-surgery modifier that should appear on the claim line. Adding modifier 80 alongside AS will trigger a claim rejection or improper payment because Medicare's processing logic treats them as contradictory signals about the assistant's credential type.

Payer rules outside Medicare vary. Many commercial plans follow Medicare's AS logic, but some require modifier 80 even for PA assistants, or they do not recognize AS at all and bundle the assistant's work into the primary surgeon's payment. Verify the specific plan's assistant-at-surgery policy before submitting, and document in the operative note that the PA, NP, or CNS was present and actively assisting throughout the procedure—not simply observing or performing unrelated tasks.

Orthopedic scenarios

Concrete situations in orthopedic practice that warrant modifier AS.

Source · Editorial brief grounded in AAOS coding guidance and cited references ↓

Total knee arthroplasty (CPT 27447): The orthopedic surgeon's PA assists with retraction, component trialing, and closure throughout the case. The PA's practice bills 27447-AS under the PA's NPI; the surgeon bills 27447 without an assistant modifier.
Open reduction and internal fixation of a distal radius fracture (CPT 25609): A nurse practitioner employed by the orthopedic group assists the surgeon. The claim submitted for the NP's services appends modifier AS to 25609.
Arthroscopic rotator cuff repair (CPT 29827): The PA assists the surgeon throughout the labral and cuff work. Modifier AS is appended to 29827 on the PA's claim. Because 29827 carries a Medicare assistant-at-surgery indicator of 1, the claim is eligible for payment at 16% of the surgeon's fee schedule amount.
Lumbar posterior spinal fusion (CPT 22630 with add-on 22632): The CNS assists during a two-level TLIF. Modifier AS is appended to both 22630 and 22632 on the CNS's claim, reflecting assistance throughout the multi-level construct.
Hip hemiarthroplasty for femoral neck fracture (CPT 27125): Surgery is performed after hours with no orthopedic resident available. The PA assists. Even though an exception for resident unavailability exists, modifier AS—not modifier 82—is correct because the assistant holds a PA credential, not an MD/DO license.
Arthroscopic ACL reconstruction with patellar tendon autograft (CPT 29888): The supervising surgeon's PA assists with graft harvest and tibial tunnel preparation. The PA bills 29888-AS. The surgeon's claim for 29888 carries no assistant modifier.

Common mistakes

Where coders most often go wrong with modifier AS.

Source · Editorial brief grounded in CMS NCCI Policy Manual and cited references ↓

Appending both modifier AS and modifier 80 to the same claim line—only AS is correct when the assistant is a PA, NP, or CNS; stacking 80 with AS triggers rejections and flags the claim for audit.
Billing modifier AS on the primary surgeon's claim line instead of on a separate claim submitted under the assisting PA's or NP's own NPI.
Using modifier AS for procedures that carry a Medicare assistant-at-surgery indicator of 0 (payment not allowed) or 9 (concept does not apply)—the claim will deny regardless of which assistant modifier is appended.
Omitting modifier AS entirely and billing the PA's assistant work under the supervising physician's NPI with no modifier, which constitutes incident-to billing for a surgical assist and does not meet Medicare's incident-to requirements.
Applying modifier AS to a teaching-hospital claim when a qualified resident was available—Medicare prohibits assistant-at-surgery payment in that scenario even if a PA was present in the OR.
Failing to document the assistant's specific intraoperative contributions in the operative report, leaving the claim vulnerable to denial on post-payment review because 'assistant at surgery' requires evidence of active participation, not mere presence.

CPT codes that use modifier AS

527 orthopedic CPT codes in our reference list this modifier as applicable. Sorted by total RVU.

Source · Derived from per-code modifier guidance in our CPT reference

Showing top 12 of 527 by total RVU.

Where modifier AS shows up

Body regions where this modifier most commonly appears in our orthopedic reference.

Spine 99 codes
Other 91 codes
Foot & ankle 66 codes
Elbow 49 codes

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 7 cited references ↓

01Can a PA bill modifier AS and modifier 80 on the same claim line?

No. Modifier AS replaces modifiers 80, 81, and 82 when the assistant is a PA, NP, or CNS. Stacking AS with 80 on the same line contradicts Medicare's processing logic and will cause a claim rejection or denial. Use only modifier AS.

02How much does Medicare pay when modifier AS is on the claim?

Medicare reimburses the PA, NP, or CNS at 85% of the assistant-at-surgery fee, which is set at 16% of the primary surgeon's Medicare Physician Fee Schedule allowable for that procedure. The effective payment is therefore roughly 13.6% of the surgeon's allowed amount.

03Does the procedure have to be pre-approved for assistant-at-surgery payment before modifier AS will work?

Yes. Every CPT code in the Medicare Physician Fee Schedule carries an assistant-at-surgery indicator. Modifier AS will not generate payment on a code with indicator 0 (payment not allowed) regardless of who assisted. Check the MPFS lookup before billing.

04Whose NPI goes on the claim when modifier AS is used?

The assisting PA's, NP's, or CNS's own NPI must appear on the claim. Billing the assistant's services under the supervising physician's NPI with modifier AS is incorrect and may constitute a false claim.

05Is modifier AS recognized by all payers, or just Medicare?

Modifier AS is a HCPCS Level II modifier primarily enforced by Medicare. Commercial and Medicaid payers vary—some mirror Medicare's AS rules, others require modifier 80 even for PA assistants, and some bundle assistant services entirely. Verify each plan's policy independently before submitting a claim with modifier AS.

06Can modifier AS be used in a teaching hospital when a resident is present?

No. Medicare prohibits payment for any assistant at surgery—PA, NP, CNS, or physician—when the procedure is performed in a teaching hospital that has an applicable training program and a qualified resident is available to assist. The teaching-hospital restriction overrides the AS modifier.

07Does the primary surgeon need to do anything differently on their claim when a PA assists?

The primary surgeon's claim for the procedure (e.g., 27447 for total knee arthroplasty) is submitted without any assistant-at-surgery modifier. The surgeon's payment is unaffected by whether a PA or a physician assistant was used. Only the assistant's separate claim carries modifier AS.

Sources & references

Editorial content was developed using the following public sources. Last verified May 8, 2026.

01CMS Medicare Claims Processing Manual, Chapter 12, §§ 50, 140 – Assistant at Surgery billing requirements
02CMS Medicare Physician Fee Schedule – Assistant at Surgery indicators (column 'Asst Surg') for each CPT code
03AAPC Knowledge Center – 'Understand How to Apply Assistant at Surgery Modifiers' (General Surgery Coding Alert and Otolaryngology Coding Alert editions)
04Novitas Solutions – Additional HCPCS Modifiers reference page (https://www.novitas-solutions.com/webcenter/portal/MedicareJH/pagebyid?contentId=00144506)
05Palmetto GBA Jurisdiction M Part B – HCPCS Modifier AS guidance (https://palmettogba.com/jmb/did/8eelfy1488)
06AMA CPT Appendix A – Modifier definitions and usage guidelines
07AAOS Global Service Data for Orthopaedic Surgery (GSD) – assistant-at-surgery context for orthopedic procedures

Mira AI Scribe

Modifier AS flags a claim line to show that a physician assistant, nurse practitioner, or clinical nurse specialist served as the assistant surgeon—not a physician or resident. It belongs on the assistant's own claim, not on the primary surgeon's claim. In orthopedic surgery, this modifier surfaces constantly: PA-assisted total knee replacements, NP-assisted arthroscopic rotator cuff repairs, CNS-assisted spinal fusions. Medicare pays 85% of the 16% assistant-at-surgery allowable when the procedure's fee-schedule indicator permits assistant payment. Never combine AS with modifier 80, 81, or 82—those are physician-only assistant modifiers. Confirm the operative note explicitly records the PA's or NP's active intraoperative role before billing.

See how Mira flags modifier AS in dictation