Modifier AS is used to identify assistant-at-surgery services provided by a Physician Assistant (PA), Nurse Practitioner (NP), or Clinical Nurse Specialist (CNS).

Unlike Modifiers 80, 81, and 82, which identify physician assistant surgeons, Modifier AS identifies services performed by qualified non-physician practitioners (NPPs) who assist the primary surgeon during an eligible surgical procedure.

Under Medicare, when a PA, NP, or CNS acts as the surgical assistant, Modifier AS must be reported together with Modifier 80, 81, or 82, as appropriate. Claims submitted with Modifier AS alone are generally returned or rejected because AS is informational and does not replace the physician assistant-at-surgery modifiers.

Modifier

Modifier Name

Physician Assistant, Nurse Practitioner, or Clinical Nurse Specialist Services for Assistant at Surgery

Note: Modifier AS is a HCPCS Level II informational modifier, not a CPT® modifier. Always follow Medicare and payer-specific billing guidelines.

Plain English Explanation

Modifier AS tells the payer:

“The assistant during surgery was a Physician Assistant (PA), Nurse Practitioner (NP), or Clinical Nurse Specialist (CNS), not another physician.”

Purpose of Modifier AS

Modifier AS is used to:

Identify non-physician practitioners serving as surgical assistants.
Differentiate PA/NP/CNS assistants from physician assistant surgeons.
Support correct Medicare reimbursement.
Ensure accurate claims processing.
Comply with CMS assistant-at-surgery billing requirements.

Understanding Modifier AS

Modifier AS applies when:

A PA, NP, or CNS actively assists the surgeon.
The surgical procedure is eligible for assistant-at-surgery payment.
Medical necessity supports assistant services.
The assistant performs medically necessary intraoperative services.
The appropriate physician assistant modifier (80, 81, or 82) is also reported when required by Medicare.

Typical duties include:

Tissue retraction.
Surgical exposure.
Hemostasis.
Suction and irrigation.
Assisting with dissection.
Suturing.
Wound closure.
Instrument handling.

Assistant-at-Surgery Indicators (Medicare)

Before billing Modifier AS, verify the Assistant at Surgery Indicator in the Medicare Physician Fee Schedule Database (MPFSDB).

Indicator	Meaning
0	Assistant-at-surgery payment generally not allowed unless documentation supports medical necessity.
1	Statutory payment restriction applies.
2	Assistant-at-surgery payment allowed when medically necessary.
9	Concept does not apply.

Always verify the indicator before billing assistant-at-surgery services.

When to Use Modifier AS

Modifier AS is appropriate when:

A PA assists the surgeon.
An NP assists the surgeon.
A CNS assists the surgeon.
The procedure allows assistant-at-surgery payment.
Medical necessity is documented.

Common Examples

✔ Total knee replacement assisted by a Physician Assistant.

✔ Laparoscopic colectomy assisted by a Nurse Practitioner.

✔ Lumbar spinal fusion assisted by a Clinical Nurse Specialist.

✔ Coronary artery bypass graft assisted by a Physician Assistant.

✔ Total hip arthroplasty assisted by an NP.

When NOT to Use Modifier AS

Do not use Modifier AS when:

The assistant is a physician (use Modifier 80, 81, or 82 as appropriate).
The procedure is not eligible for assistant-at-surgery payment.
A surgical technologist or first assistant who is not a PA, NP, or CNS provides assistance.
The service is not medically necessary.
The payer does not recognize Modifier AS (follow payer-specific instructions).

Medicare Rules

CMS rules include:

Modifier AS identifies assistant-at-surgery services performed by a PA, NP, or CNS.
Modifier AS should be reported with Modifier 80, 81, or 82, depending on the circumstances.
Claims submitted with Modifier AS alone are generally returned because Medicare requires both modifiers.
Medicare pays eligible PA/NP/CNS assistant-at-surgery services at 85% of the physician assistant-at-surgery fee, which is effectively 13.6% of the Medicare Physician Fee Schedule amount for eligible procedures.
Payment is allowed only if the CPT® code is eligible for assistant-at-surgery reimbursement and medical necessity is documented.

Commercial Insurance Rules

Commercial payer policies vary.

Many insurers:

Recognize Modifier AS.
Require PA/NP/CNS credentialing.
Require operative reports.
Follow Medicare assistant-at-surgery concepts.
May allow AS without additional modifiers depending on payer policy.

Always verify payer-specific billing guidelines before claim submission.

Documentation Requirements

Documentation should include:

Date of surgery.
CPT® procedure performed.
Primary surgeon’s name.
PA/NP/CNS assistant’s name.
Professional credentials.
Description of assistant services.
Medical necessity.
Operative report.
Provider signatures.

The operative report should clearly describe the assistant’s participation throughout the applicable portion of the surgery.

Real Billing Examples

Example 1 – Total Knee Arthroplasty

An orthopedic surgeon performs a total knee replacement.

A Physician Assistant assists throughout the operation.

Billing

Primary Surgeon:

27447

PA Assistant:

27447-80-AS (Medicare billing example)

Example 2 – Laparoscopic Colectomy

A Nurse Practitioner assists during a laparoscopic colectomy.

Modifier AS is reported according to payer requirements, along with the applicable assistant-at-surgery modifier when required by Medicare.

Example 3 – Lumbar Fusion

A Clinical Nurse Specialist assists with spinal instrumentation and wound closure.

Modifier AS is appropriate if the procedure qualifies for assistant-at-surgery payment.

Example 4 – Incorrect Use

A surgical technologist assists during surgery.

Modifier AS should not be reported, because CMS limits AS to qualified PA, NP, or CNS providers.

CMS-1500 Claim Example

Field	Example
CPT Code	27447
Modifiers	80, AS
Diagnosis Pointer	Appropriate ICD-10-CM diagnosis
Units	1
Charges	Assistant provider’s billed amount

Billing Example

Provider	CPT® Code
Primary Surgeon	27447
Physician Assistant	27447-80-AS

Common Denial Reasons

Modifier AS billed without Modifier 80, 81, or 82 for Medicare.
CPT® code not eligible for assistant-at-surgery payment.
Provider not credentialed.
Medical necessity not documented.
Missing operative report.
Assistant provider is not a PA, NP, or CNS.

How to Correct the Denial

Review the EOB or ERA.
Verify assistant-at-surgery eligibility.
Confirm the provider is a PA, NP, or CNS.
Append the appropriate assistant-at-surgery modifier with Modifier AS for Medicare.
Submit the operative report and supporting documentation.
Appeal with complete medical records if appropriate.

Coding Tips

Modifier AS is a HCPCS Level II modifier.
Verify the Assistant at Surgery Indicator before billing.
Under Medicare, report AS together with 80, 81, or 82 as appropriate.
Do not use AS for physician assistant surgeons (MD/DO).
Verify payer-specific modifier sequencing requirements.

Modifier AS vs Modifier 80 vs Modifier 81 vs Modifier 82

Modifier	Purpose
80	Physician assistant surgeon throughout the procedure.
81	Physician providing minimum assistant-at-surgery services.
82	Physician assistant surgeon in a teaching hospital when no qualified resident is available.
AS	PA, NP, or CNS assistant-at-surgery services.

Modifier AS vs Modifier 62

Modifier	Purpose
AS	PA, NP, or CNS assists the primary surgeon.
62	Two physicians each perform distinct portions of the surgery as co-surgeons.

Frequently Asked Questions (FAQs)

Q1. What is Modifier AS?

Answer: Modifier AS identifies assistant-at-surgery services provided by a Physician Assistant, Nurse Practitioner, or Clinical Nurse Specialist.

Q2. Can a physician bill Modifier AS?

Answer: No. Modifier AS is intended only for PA, NP, or CNS assistant-at-surgery services. Physicians use Modifiers 80, 81, or 82 instead.

Q3. Can Modifier AS be billed alone to Medicare?

Answer: Generally, no. Medicare requires Modifier AS to be reported with Modifier 80, 81, or 82. Claims with AS alone are typically returned.

Q4. How much does Medicare pay for Modifier AS?

Answer: Medicare generally reimburses eligible PA/NP/CNS assistant-at-surgery services at 85% of the physician assistant-at-surgery payment, resulting in approximately 13.6% of the MPFS allowable amount for eligible procedures.

AR Caller Tips

When following up on a denied Modifier AS claim:

Verify the assistant is credentialed as a PA, NP, or CNS.
Confirm the CPT® code is eligible for assistant-at-surgery payment.
Ensure Modifier AS is paired with Modifier 80, 81, or 82 for Medicare claims.
Request review of the operative report.
Document the payer representative’s name, reference number, and appeal instructions.

Interview Questions

Question 1

What is Modifier AS?

Answer: Modifier AS reports assistant-at-surgery services performed by a Physician Assistant, Nurse Practitioner, or Clinical Nurse Specialist.

Question 2

Can Modifier AS be billed without another assistant-at-surgery modifier for Medicare?

Answer: No. Medicare generally requires Modifier AS to be billed with Modifier 80, 81, or 82.

Question 3

Who can report Modifier AS?

Answer: Only qualified Physician Assistants, Nurse Practitioners, and Clinical Nurse Specialists providing assistant-at-surgery services.

Practice Scenario

Scenario

A patient undergoes a total hip arthroplasty. A credentialed Physician Assistant assists throughout the operation with surgical exposure, retraction, instrumentation, and wound closure. The procedure has an Assistant at Surgery Indicator of 2 in the MPFSDB.

Question

Should Modifier AS be reported?

Answer

Yes. The PA qualifies as an assistant at surgery, the procedure is eligible, and documentation supports medical necessity. For Medicare, the assistant’s claim should include Modifier AS together with the applicable assistant-at-surgery modifier (for example, 80-AS when appropriate).

Related Modifiers

Modifier 80 – Assistant Surgeon
Modifier 81 – Minimum Assistant Surgeon
Modifier 82 – Assistant Surgeon (When Qualified Resident Surgeon Not Available)
Modifier 62 – Two Surgeons
Modifier 66 – Surgical Team

Common Billing Mistakes

Billing AS without the required companion modifier for Medicare.
Using AS for physician assistants (MD/DO) instead of PA/NP/CNS providers.
Billing AS for surgical technologists.
Failing to verify assistant-at-surgery eligibility.
Omitting documentation of medical necessity.

Key Takeaways

Modifier AS identifies assistant-at-surgery services performed by a PA, NP, or CNS.
Under Medicare, AS is generally reported with Modifier 80, 81, or 82.
Verify the Assistant at Surgery Indicator before billing.
Medicare generally reimburses eligible PA/NP/CNS assistant-at-surgery services at approximately 13.6% of the MPFS allowable amount.
Thorough operative documentation is essential for payment.

References

CMS Medicare Claims Processing Manual – Assistant at Surgery Billing Requirements.
CMS Recovery Audit Program – Non-Physician Assistant at Surgery Modifier AS.
CMS Recovery Audit Program – Assistant at Surgery Services.
AMA CPT® Professional Edition (licensed codebook).

Conclusion

Modifier AS is an important HCPCS modifier used to identify Physician Assistant, Nurse Practitioner, and Clinical Nurse Specialist services provided as assistant at surgery. Correct reporting requires confirming assistant-at-surgery eligibility, documenting medical necessity, and following Medicare’s requirement to pair Modifier AS with the appropriate assistant-at-surgery modifier when applicable. Proper use helps ensure compliant billing, appropriate reimbursement, and fewer claim denials.

Educational Disclaimer

This article was prepared with the assistance of artificial intelligence (AI) for educational and informational purposes. It is based on publicly available CMS guidance, Medicare billing resources, and general medical billing principles. It is not an official publication of the American Medical Association (AMA), CMS, or any insurance payer. CPT® is a registered trademark of the American Medical Association. Always consult the latest AMA CPT® codebook, CMS manuals, Medicare Administrative Contractor (MAC) guidance, the Medicare Physician Fee Schedule Database, National Correct Coding Initiative (NCCI) policies, and payer-specific billing policies before coding, billing, or submitting claims.