Modifier AS: PA, NP and CNS Assistant at Surgery
The AS modifier bills a PA, NP or CNS who assisted at surgery. Medicare allows 85% of the 16% physician rate, or 13.6% of the surgery. Rules and AS vs 80.
Modifier AS is a HCPCS Level II modifier for assistant-at-surgery services furnished by a physician assistant (PA), nurse practitioner (NP) or clinical nurse specialist (CNS). Medicare's fee schedule amount for these services is 85% of what a physician who assists at surgery receives, and since physicians get 16% of the surgical amount, AS services come to 13.6% of it (Pub. 100-04, ch. 12, §§110.2 and 120.1).
Key takeaways
- AS is required whenever a PA, NP or CNS bills assistant-at-surgery services. Don't use 80, 81 or 82 for them.
- Payment is 85% of the 16% a physician assisting at surgery receives: 13.6% of the surgical fee schedule amount.
- The same assistant-at-surgery indicator rules apply: the code must allow an assistant.
- The PA, NP or CNS must actively assist and do more than ancillary services, within their State scope of practice.
- Teaching hospital limits on assistants apply to AS claims too.
Modifier AS · payment effect
With and without the modifier
27447 without AS · national facility
$1,159.35
Total knee replacement
27447-AS · NP/PA/CNS assistant: 13.6%
$157.67
85% of the 16% assistant-at-surgery amount.
When to use the AS modifier
Use AS on the surgical code when:
- A PA, NP or CNS actively assisted the surgeon and furnished more than ancillary services (ch. 12, §§110.2 and 120.1).
- State law authorizes them to serve as an assistant at surgery (ch. 12, §100.1.7.B).
- The procedure's assistant-at-surgery indicator allows payment. Indicator 2 means payable, 0 needs documentation of medical necessity, and 1 means an assistant is never paid. Check the indicator on the code page before scheduling the claim.
Example. A PA first-assists on a total knee replacement (27447, indicator 2). The surgeon bills 27447; the PA's claim (under the PA's own NPI or reassigned to the group) bills 27447-AS.
CMS payment indicators · 27447
Total knee replacement
| Rule | CMS value | What it means |
|---|---|---|
| Global period | 090 | Major surgery: the day before, the day of, and 90 days after are included. |
| Multiple procedures | 2 | Standard reduction: highest-valued procedure at 100%, others at 50%. |
| Bilateral (modifier 50) | 1 | Bilateral with modifier 50 pays 150% of the fee schedule amount. |
| Assistant at surgery (80/81/82/AS) | 2 | Paid. |
| Co-surgeons (62) | 1 | Permitted with supporting documentation. |
| Team surgery (66) | 0 | Not permitted. |
| Professional/technical | 0 | Physician service: no professional/technical split. |
| Split (54/55/56) | 0.10/0.69/0.21 | Share of the payment for pre-op, the procedure itself, and post-op care. |
When not to use the AS modifier
- The assistant is a physician. Use 80, 81 or 82.
- The PA or NP performed the procedure as the primary surgeon within their scope. Bill the code without AS; it's paid at the NPP rate.
- Teaching hospitals where a qualified resident was available. Medicare doesn't pay assistants at surgery there, except under the exceptions in §100.1.7.
- Commercial claims without checking policy. Other payers set their own assistant-at-surgery rules.
How the AS modifier affects payment
The fee schedule amount for an AS line is 85% of the amount for a physician assisting at surgery, and that amount is 16% of the surgery's fee schedule amount:
Medicare then pays 80% of the lesser of the actual charge or that amount, as with other Part B services. When several procedures are performed, the assistant's lines are ranked and reduced under the multiple-procedure rules like the surgeon's (modifier 51).
AS vs 80, 81 and 82
| Modifier | Assistant | Medicare allowed amount |
|---|---|---|
| AS | PA, NP or CNS | 13.6% of the surgical amount (85% of 16%) |
| 80 | Physician, full assistance | 16% of the surgical amount |
| 81 | Physician, minimal assistance | 16% of the surgical amount |
| 82 | Physician in a teaching hospital, no qualified resident available | 16% of the surgical amount |
FAQ
How much does Medicare pay for an assistant surgeon?
A physician who assists at surgery is allowed 16% of the surgery's fee schedule amount. A PA, NP or CNS billing with AS is allowed 85% of that, which is 13.6% of the surgical amount.
What is the AS modifier used for?
To bill assistant-at-surgery services furnished by a physician assistant, nurse practitioner or clinical nurse specialist. CMS requires AS on these claims.
Can a nurse practitioner bill modifier 80?
No. Modifiers 80, 81 and 82 are for physicians. NPs, PAs and CNSs bill assistant-at-surgery services with AS.
Is an assistant surgeon a doctor?
It can be either. A physician who assists bills 80, 81 or 82. A PA, NP or CNS authorized by State law can also be an assistant at surgery and bills AS.
Does the assistant-at-surgery indicator apply to AS claims?
Yes. AS claims follow the same indicator rules as physicians who assist: payable for indicator 2, documentation needed for 0, and never paid for 1.
Keep reading
- Modifier 80 for physicians who assist at surgery.
- Modifier 51 for multiple-procedure reductions.
- Global period calculator for the surgery's global window.
- Codes on this page: 27447 64721 27130
Sources: CMS Medicare Claims Processing Manual, Pub. 100-04, ch. 12, §§20.4.3, 100.1.7, 110.2 and 120.1. Verified October 6, 2026.
