Glossary · Coding

Modifier AS (PA/NP/CNS assistant)

Modifier AS is a HCPCS Level II modifier appended to a procedure code when a physician assistant (PA), nurse practitioner (NP), or clinical nurse specialist (CNS) serves as the first assistant at surgery; Medicare reimburses the assisting non-physician practitioner at 85% of the physician assistant-at-surgery allowable, which works out to roughly 13.6% of the primary surgeon's fee schedule amount.

Verified May 8, 2026 · 4 sources ↓

Drawn from AAOSEzsettlesolutionsCMS

Definition

Source · Editorial summary grounded in 4 cited references ↓

Modifier AS signals to a payer—most importantly Medicare—that the assistant at surgery is a non-physician practitioner (NPP): a physician assistant, nurse practitioner, or clinical nurse specialist. It is a HCPCS Level II modifier, not a CPT modifier, so it lives outside the standard CPT codebook and is governed by CMS policy rather than AMA guidance. The modifier is placed on the same procedure code billed by the surgeon (or on a separate claim line for the assistant's fee), identifying the NPP's role without altering the procedure's definition.

Reimbursement math matters here. Medicare pays physician assistants-at-surgery 16% of the surgeon's fee schedule amount when the assistant is a physician (modifier 80). When the assistant is an NPP using modifier AS, Medicare applies its 85% NPP payment reduction on top of that assistant-at-surgery rate, landing at approximately 13.6% of the primary surgeon's allowable. The AAOS Resident Guide summarizes this as 'AS pays 15% less than physician allowable.' Not all procedures are eligible—Medicare only covers an assistant at surgery when the procedure is on the approved assistant-at-surgery list; if the procedure code carries a '0' restriction (no assistant-at-surgery covered), the claim will deny regardless of which assistant modifier is appended.

Private payers are not bound by Medicare's AS rules. Some follow Medicare logic closely; others have proprietary policies that may pay a flat percentage, require pre-authorization, or decline NPP assistant coverage altogether. Always verify each payer's assistant-at-surgery policy before billing.

Why it matters

Using the wrong assistant-at-surgery modifier—for example, appending modifier 80 (physician assistant surgeon) to a claim when the actual assistant was a PA or NP—constitutes a misrepresentation of the provider type that performed the service. Medicare can recoup the overpayment and flag the claim for audit. Conversely, failing to append modifier AS and billing nothing means the practice absorbs the cost of the NPP's time in the OR. Getting the modifier right also requires confirming that the procedure code itself is eligible for assistant-at-surgery reimbursement; submitting AS on an ineligible code results in a predictable denial that wastes billing cycles and delays cash flow.

Common mistakes

Where people most often go wrong with this concept.

Source · Editorial brief grounded in cited references ↓

Appending modifier 80 instead of AS when the assistant is a PA, NP, or CNS—this misidentifies the provider type and overstates the reimbursement rate.
Billing modifier AS on procedure codes that Medicare has designated as not payable with an assistant at surgery (assistant-at-surgery restriction code '0'), guaranteeing a denial.
Assuming all private payers reimburse modifier AS at the same rate or under the same rules as Medicare—many do not, and unchecked payer-specific policies lead to systematic underpayment or claim rejections.
Failing to document the NPP's presence and active participation in the operative report; payers can deny or recoup if the record does not support the assistant role.
Confusing modifier AS with modifier 82 (assistant surgeon when a qualified resident is unavailable), which applies only to physician assistants-at-surgery in teaching facilities, not to NPs or CNSs.
Omitting the NPP's individual NPI on the claim line for the assistant's service, causing the claim to route incorrectly or deny for missing provider information.

Related codes

Codes commonly involved when this concept appears in practice.

CPT

Modifiers

80 Assistant surgeon

Frequently asked questions

Source · Generated from the editorial pipeline, verified against 4 cited references ↓

01What is the difference between modifier AS and modifier 80?

Modifier 80 identifies a physician (MD or DO) serving as the assistant at surgery and is reimbursed at roughly 16% of the primary surgeon's Medicare allowable. Modifier AS identifies a non-physician practitioner—PA, NP, or CNS—in that same role and is reimbursed at approximately 13.6% of the primary surgeon's allowable because Medicare applies its 85% NPP payment reduction on top of the assistant-at-surgery rate.

02Can a CNS bill modifier AS for orthopedic procedures?

Yes. CMS groups clinical nurse specialists with physician assistants and nurse practitioners under the modifier AS umbrella. The CNS must be acting as the first assistant at surgery, and the procedure must be on Medicare's approved assistant-at-surgery list for the claim to be payable.

03How do I know if a CPT code allows an assistant at surgery under Medicare?

Each CPT code in the Medicare Physician Fee Schedule database carries an assistant-at-surgery indicator. A value of '0' means no assistant coverage is allowed; '1' means allowed by exception with documentation; '2' means allowed. Check the CMS Physician Fee Schedule Look-Up Tool or your MAC's local coverage policy before billing modifier AS.

04Do private payers follow Medicare's AS payment rules?

Not automatically. Many commercial payers have their own assistant-at-surgery fee schedules and may require prior authorization for the NPP assistant role, pay a flat contracted rate, or exclude NPP assistants altogether. Verify each payer's policy before scheduling an NPP as the surgical assistant.

05What documentation is required to support a modifier AS claim?

The operative report must identify the PA, NP, or CNS by name and credential, describe their active participation in the procedure (not just observation), and be signed by the primary surgeon. 'Assisted by [NPP name]' alone is often insufficient; the note should reflect the nature of the assistance provided.

Sources & references

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

01
aaos.org
https://www.aaos.org/globalassets/quality-and-practice-resources/coding-and-reimbursement/resident-guide/resident-guide_modifiers.pdf
02
ezsettlesolutions.com
https://ezsettlesolutions.com/a-comprehensive-guide-to-orthopedic-medical-billing-modifiers/
03CMS Medicare Physician Fee Schedule: Assistant-at-Surgery Indicator — https://www.cms.gov/Medicare/Medicare-Fee-for-Service-Payment/PhysicianFeeSched
04CMS MLN: Non-Physician Practitioners — https://www.cms.gov/Outreach-and-Education/Medicare-Learning-Network-MLN/MLNProducts/MLN-Publications-Items/ICN901623

Mira AI Scribe

When Mira detects that an operative note identifies a PA, NP, or CNS as the first assistant at surgery, it will prompt the coder to append modifier AS to the relevant procedure code on the assistant's claim and verify three things before submission: (1) the procedure code is on Medicare's approved assistant-at-surgery list for the applicable MAC jurisdiction; (2) the operative report explicitly names the NPP and describes their active role—not merely their presence—in the procedure; and (3) the payer on the claim is Medicare or a plan with a confirmed AS-compatible policy. If the procedure code carries an assistant-at-surgery restriction that precludes coverage, Mira will flag the line for coder review rather than auto-appending the modifier. For split or shared surgical cases where a physician and an NPP both participate, Mira will surface a distinct alert noting that the assistant billing rules differ from the split/shared E/M rules and that separate documentation standards apply.

See Mira's approach