A bill from an assistant surgeon you never met: the 16 percent rule and the fee-file code behind it

The fee file that holds those amounts also holds a one-digit code for every procedure, the assistant-at-surgery indicator, and that digit decides whether anyone is paid at all. In the October 2026 relative value file, 4,178 active codes carry a 10-day or 90-day surgical global period. Of those, 2,442 carry indicator 2, which lets an assistant be paid, 543 carry indicator 0, which needs documentation of medical necessity, and 1,193 carry indicator 1, a restriction the file labels statutory. Knowing which digit your operation carries tells you whether the line on your bill should exist, and the percentage rule tells you how large it should be.

The surgeon's fee has no room for a second person

Medicare defines a global surgical package for every major and minor operation, and chapter 12 of the Claims Processing Manual, section 40.1, lists what the surgeon's approved amount includes: visits before the operation, the intra-operative services that are a usual and necessary part of the procedure, complications that do not require a return to the operating room, follow-up visits during the recovery period, post-surgical pain management by the surgeon, supplies and routine dressing changes. The manual's wording matters here. The amount covers those services "when furnished by the physician who performs the surgery." A second clinician who steps into the operation is not the physician who performs it, so nothing in the surgeon's amount pays for that work. How long the package runs after the operation, and which follow-up visits can still be billed on top of it, is the subject of the global period follow-up visit guide.

The second clinician therefore bills on their own claim, under their own identifier, and the claim usually repeats the procedure code the surgeon billed. UnitedHealthcare's published assistant policy says so directly: an assistant should submit the identical procedure code as the primary surgeon with a modifier. On paper, the assistant's line reads like a copy of the operation you already saw charged, at a smaller price, from a name you do not recognize. A surgical day can produce several bills from people who are not the surgeon: the anesthesia clinician, a pathologist who reads any tissue, the facility, and sometimes an assistant. The anesthesia billing guide works through the first of those, which follows the same logic of a separate claim for work done while the patient is asleep.

The manual defines who qualifies. In section 100.1.7 of chapter 12, an assistant at surgery is "a physician who actively assists the physician in charge of a case in performing a surgical procedure," and a nurse practitioner, physician assistant or clinical nurse specialist authorized under state law can also serve. For physician assistants and for nurse practitioners and clinical nurse specialists, sections 110.2 and 120.1 add a test: the person must actively assist the physician and furnish more than just ancillary services. Being in the room is not enough on its own; the claim has to describe active assistance in the procedure.

Three modifiers carry the claim. The manual refers to modifiers 80, 81 and 82 as the assistant-at-surgery physician modifiers, and to the AS modifier for physician assistants, nurse practitioners and clinical nurse specialists. Section 100.1.7, which deals with teaching hospitals, describes 82 as the modifier indicating that a qualified resident surgeon was not available. A claim for a procedure with any of these modifiers is paid, according to section 20.4.3, only if the services of an assistant at surgery are authorized. That phrase leads to the fee file.

A one-digit code in the fee file decides whether anyone is paid

The national relative value file, which CMS publishes every year with the Physician Fee Schedule, has a column headed ASST SURG. Its documentation defines the four values in these words: 0 is a "payment restriction for assistants at surgery" that applies "unless supporting documentation is submitted to establish medical necessity"; 1 is a "statutory payment restriction" under which an assistant "may not be paid"; 2 means the restriction "does not apply" and an assistant "may be paid"; and 9 means the concept does not apply. The Claims Processing Manual, in its section on denied assistant claims, describes codes with an indicator of 0 or 1 as procedures for which CMS has determined that an assistant surgeon is not generally medically necessary.

Two legal provisions sit behind indicator 1. Section 1862(a)(15) of the Social Security Act, in 42 U.S.C. 1395y, excludes from Part B coverage the services of an assistant at surgery in a cataract operation, including the later insertion of an intraocular lens, unless the quality improvement organization or the carrier approved the assistant before the surgery because of a complicating medical condition. It also excludes assistant services in any procedure covered by section 1848(i)(2)(B), the so-called five percent test: if the national average share of a procedure performed with a physician as assistant at surgery is under 5 percent, no payment may be made for the assistant. The regulation at 42 CFR 411.15(n) repeats both exclusions in the same terms. Indicator 0 reflects a CMS determination that the procedure does not generally need an assistant, and it is the one the file says documentation can lift.

Source: CMS Physician Fee Schedule relative value file PPRRVU2026_Oct_nonQPP (released 08/26/2026, inside rvu26d-updated-08-26-2026.zip), lines without a modifier, ASST SURG column; indicator definitions from the RVU26D documentation. Counted by script on November 3, 2026. The "all lines" column includes codes that Medicare does not pay under the fee schedule, which is where most of the 9s sit.

The first column of numbers is the population that matters for an operating-room bill: codes with active fee schedule status and a global period of 10 or 90 days. Among those, 2,442 allow an assistant, and the other 1,736 do not allow one by default. The 1,193 codes under indicator 1 outnumber the 543 under indicator 0, which means the fee file treats most restricted codes as closed to assistants by statute rather than by a documentation test. A code's place in the list says nothing about any one patient's operation. The 90-day codes tell the more relevant story for large operations: 2,416 of the 3,719 carry indicator 2, while the 10-day minor procedures lean the other way, with 333 of 459 under indicator 1.

Indicator 9 is the least intuitive entry, because it appears on a CT scan, a psychiatric evaluation and many drug and supply codes where an assistant is not a concept at all. An assistant-at-surgery modifier on a line with such a code is a mismatch to ask about, not a coverage question.

Sixteen percent, then eighty-five percent of that

The amount paid to a physician who assists is set in the statute. Section 1848(i)(2)(A) of the Social Security Act provides that for a surgical service furnished by a physician, if payment is made separately for the services of a physician serving as an assistant at surgery, the fee schedule amount "shall not exceed 16 percent of the fee schedule amount otherwise determined under this section for the global surgical service involved." Chapter 12, section 20.4.3 of the Claims Processing Manual states it as an instruction: the fee schedule amount for an assistant-at-surgery physician "equals 16 percent of the amount otherwise applicable for the surgical payment."

Physician assistants, nurse practitioners and clinical nurse specialists are paid under a different clause. Section 1833(a)(1)(O) of the Act, in 42 U.S.C. 1395l, says that for those practitioners' services the amount paid is 80 percent of the lesser of the actual charge or 85 percent of the fee schedule amount, and for services as an assistant at surgery, 80 percent of the lesser of the actual charge or "85 percent of the amount that would otherwise be recognized if performed by a physician who is serving as an assistant at surgery." The regulations agree: 42 CFR 414.52(d) for physician assistants and 414.56(c) for nurse practitioners and clinical nurse specialists both cap the allowed amount for assistant-at-surgery services at 85 percent of the physician amount. Sections 110.2 and 120.1 of chapter 12 close the loop with arithmetic: since physicians are paid 16 percent of the surgical payment, the amount for these practitioners is 13.6 percent.

The 80 percent in the statute is the usual Part B split, not an extra reduction. Medicare pays 80 percent of the approved amount, and the patient owes the other 20 percent as coinsurance once the Part B deductible is met. The coinsurance figures below assume the deductible is already met, which is an assumption and not a fact about any reader. The chain for a total hip replacement shows each step with the surgeon amount first:

Source: CMS PFS relative value file PPRRVU2026_Oct_nonQPP and GPCI2026 (Detroit, Michigan Locality 01: work 1.000, practice expense 0.965, malpractice 1.686), $33.4009 conversion factor; Medicare Claims Processing Manual chapter 12, sections 20.4.3, 110.2 and 120.1 (Rev. 13753); 42 U.S.C. 1395w-4(i)(2) and 1395l(a)(1)(O). Computed November 3, 2026. Medicare anchors, not your bill.

Working the first line by hand: the work part is 19.11 x 1.000 = 19.11000, the practice expense part is 11.63 x 0.965 = 11.22295, and the malpractice part is 4.05 x 1.686 = 6.82830. Those add to 37.16125, and 37.16125 x $33.4009 is $1,241.22. Sixteen percent of that is $198.60, and 85 percent of 16 percent is 13.6 percent, which is the $168.81 on the fifth line. The two practitioner rates differ by $29.79, and the coinsurance gap between them is $5.96.

For a PA, NP or CNS, the statute pays 80 percent of "the lesser of the actual charge" and the computed amount, so an assistant who bills less than the fee schedule amount is paid on the lower number. The percentage itself is taken on the amount actually applicable to the surgery. Each figure here is rounded to the cent from the surgeon amount; a claims system that takes 85 percent of an already rounded 16 percent amount can land a cent away when a half cent turns up, and the knee arthroscopy row below is the example: 85 percent of $86.30 is $73.355, so a system that rounds at that step can show $73.35 or $73.36, and the table uses $73.36, the direct 13.6 percent figure.

Six common operations, priced as an assistant's claim

Surgeons' amounts for these operations are facility amounts: none of the six codes has a nonfacility payment in the file, and the file's documentation marks such a gap as a procedure rarely or never performed outside a facility. The table prices each operation's assistant claim at both rates. For the two codes where the indicator does not say "paid," the amounts are what the assistant would be allowed if documentation or advance approval cleared the restriction, and they are shown so the size of the claim is visible, not because it is payable by default.

Source: CMS PFS relative value file PPRRVU2026_Oct_nonQPP (facility practice expense and total; ASST SURG column) and GPCI2026, Michigan Locality 01, CY2026, $33.4009 conversion factor. 13.6 percent computed directly from the surgeon amount. Another locality's amounts differ. Computed November 3, 2026. Medicare anchors, not your bill.

Show the numbers behind this figure

CPT	Indicator	Assistant physician, 16%	PA, NP or CNS, 13.6%
29881 Knee arthroscopy with meniscectomy	0	$86.30	$73.36
49505 Open inguinal hernia repair, age 5 or older, reducible	2	$87.79	$74.62
47562 Laparoscopic gallbladder removal	2	$109.77	$93.30
27130 Total hip replacement	2	$198.60	$168.81
22612 Posterior lumbar spinal fusion, one interspace	2	$256.40	$217.94
66984 Cataract removal with lens implant, routine	1	$74.85	$63.62

The amounts run from $74.85 for a cataract operation to $256.40 for a one-level lumbar fusion, a spread that follows the surgeon amounts of $467.80 and $1,602.51. Among the four codes with indicator 2, the hernia repair (49505) comes to $87.79 and the gallbladder removal (47562) to $109.77. Those figures give a patient holding an assistant's bill a scale to hold it against. The billed charge is whatever the practice listed, the Medicare amount is a published anchor for one locality, and what a commercial plan allows is set by its own contract, which is the subject of a later section.

The knee arthroscopy row is the one that splits the two systems. CPT 29881 carries indicator 0, so Medicare's file starts from a position that an assistant is not generally needed, while a plan that pays only indicator 2 codes, as the UnitedHealthcare policy below does, denies the line outright. The knee arthroscopy price page uses the same $539.38 surgeon amount, published in the Michigan Locality 01 fee schedule file from the regional contractor, and the cataract page uses $467.80; the match between those figures and the ones computed here is a useful cross-check on the method. For the other side of a day-surgery price, the facility layer, the guide on surgery center versus hospital outpatient cost explains why the surgeon's amount does not change with the building.

When the second name on the operative claim is not an assistant

Spinal fusion shows the most expensive confusion. CPT 22612 has an assistant indicator of 2 and also a co-surgeon indicator of 2, which the file defines as co-surgeons permitted with no documentation required when the two-specialty requirement is met. The file describes the co-surgeon indicator as marking services for which two surgeons, each in a different specialty, may be paid, and Medicare treats that arrangement as something other than assisting. Section 40.8 of chapter 12 says that when the individual skills of two surgeons are required, "the additional physicians are not acting as assistants-at-surgery." Each surgeon bills the procedure with modifier 62, and each is paid 62.5 percent of the global surgery fee schedule amount.

On the lumbar fusion that is $1,001.57 to each surgeon, set against $256.40 for an assistant, a difference of $745.17 on the second person's line. The modifier on the bill therefore changes the approved amount by that much, before any coinsurance. The manual adds a rule that protects against double counting: payment is not generally allowed for an assistant surgeon when payment for co-surgeons or team surgeons is appropriate, and a bill for an assistant after co-surgeon payment is paid only if review verifies medical necessity. A bill that shows both a 62 line and an 80 line for the same operation deserves a question.

Cataract surgery is the case where the statute, not the schedule, sets the limit. The indicator of 1 on CPT 66984 reflects the Social Security Act exclusion described earlier, and the same section carves out an exception: an assistant is covered if the quality improvement organization or the carrier approved the assistant before the surgery because of a complicating medical condition. That is an advance approval, not something a billing office can supply afterward. If a cataract operation produces an assistant line, the useful question is whether an approval existed before the date of surgery, and the answer is either a document or its absence.

Teaching hospitals add a third layer. Under 42 CFR 415.190, which applies to hospitals with an approved residency program, fee schedule payment is not available for an assistant at surgery when the hospital has a training program in the specialty and a resident is available, except in listed circumstances: exceptional medical circumstances, a complex procedure performed by a team of physicians, concurrent care by a physician of another specialty, an assistant primarily engaged in surgery when the primary surgeon does not use interns and residents, or a procedure in which CMS finds assistants are used less than 5 percent of the time. The manual's alternative route is modifier 82 or a written certification that no qualified resident was available, with the A/B contractor retaining the claim and certification for four years and reviewing after payment. The same section says assistant claims denied under these instructions do not qualify for payment under the limitation on liability provision. What that means for who may bill the patient is not spelled out in that section, so a patient with an 82 line or a denied teaching-hospital line is better served by asking the contractor or plan than by assuming.

What an assistant may bill a Medicare patient

The manual states in section 20.4.3 that Medicare's policies on billing patients in excess of the Medicare allowed amount apply to assistant-at-surgery services, and that physicians who knowingly and willfully bill a beneficiary for an assistant-at-surgery service in violation of those policies may face penalties under section 1842(j)(2) of the Act. Section 40.8 of the same chapter ties the denial messages to the indicator, and the messages are the part a patient can see.

Sources: Medicare Claims Processing Manual chapter 12, sections 20.4.3 and 40.8 (Rev. 13753); X12 claim adjustment reason code list (CARC 54: "Multiple physicians/assistants are not covered in this case"). Checked November 3, 2026.

The indicator 0 row is where the Advance Beneficiary Notice guide becomes relevant, because limitation on liability is the Medicare concept that decides whether a patient or a provider bears a denied amount when the patient did not receive a valid notice before the service. Whether it applied to a particular assistant's line depends on facts on that claim, and the Medicare Summary Notice for the date is the document to read. The outpatient procedure guide shows how the same 20 percent and the Part B deductible apply to the surgeon's and facility's lines on the same operation.

One insurer's published rule, set beside Medicare's

Commercial plans do not have to follow Medicare, and what they pay an assistant surgeon is written into contracts that patients cannot see. Some publish their policies. UnitedHealthcare's Commercial Reimbursement Policy 2026R5000D, "Assistant-at-Surgery Services Policy, Professional" (history dated through June 28, 2026), is a readable example of how a large plan builds on the Medicare file, and the differences from Medicare are exactly the ones that surprise patients. It is one insurer's policy for its commercial plans, and it says on its face that provider contracts, benefit documents and mandates can supersede it.

Sources: UnitedHealthcare Commercial Reimbursement Policy 2026R5000D, Assistant-at-Surgery Services Policy, Professional; Medicare Claims Processing Manual chapter 12 sections 20.4.3, 100.1.7, 110.2 and 120.1; 42 CFR 415.190. Checked November 3, 2026. One insurer's published policy, not a statement about commercial plans in general.

The base is the most important difference. Medicare's 16 percent applies to a published fee schedule amount, which is why dollar anchors can be printed for it. The insurer's 16 percent applies to the plan's own allowed amount for the surgeon, a number that is not public and varies by contract and by network status. The only honest way to show the effect is an input stated in advance. Suppose a plan's allowed amount for the primary surgeon is $3,000.00, a round number chosen for the arithmetic, not a price anyone was quoted. Sixteen percent is $480.00 for an assistant physician and 14 percent is $420.00 for a PA, NP or CNS. A patient with a 20 percent coinsurance and the deductible already met would owe $96.00 or $84.00 on those lines if the assistant is in network. The 14 percent rate also shows that a plan can allow a non-physician slightly more than Medicare's 13.6 percent, so Medicare's number is a point of comparison, not a limit on what a plan pays.

The eligible-list rule produces the sharpest contrast. For knee arthroscopy (indicator 0), Medicare can pay the $86.30 assistant claim when documentation establishes medical necessity, and a plan that pays only indicator 2 codes pays nothing on that line. Reading the EOB against the plan's own policy, which some insurers post for providers, is how a patient finds out which rule applied.

Assistant surgeons also appear in federal balance-billing rules. They are among the services for which the notice-and-consent exception to the No Surprises Act cannot be used, so an out-of-network assistant at an in-network facility is covered by the rule's balance-billing limits. The step-by-step decision tree for that situation is in the guide to No Surprises Act protections when you have insurance, which covers that question in full. The question that comes first is a different one: whether the assistant's line should exist at all, and how large the plan's allowed amount for it should be.

Reading the claim line and the EOB against the file

Start with three facts printed on the bill: the procedure code, the modifier and the name of the person billing. Then add two from the explanation of benefits: the allowed amount and the patient responsibility. The EOB reading guide explains the columns. Everything below is a billing check on a document, and none of it says that anyone's presence in the operating room was or was not needed.

To find the indicator, search the CPT code in the CMS Physician Fee Schedule Look-Up Tool. CMS says the tool is the place to check whether payment policy affects "payment of assistant at surgery services" for a code, and the relative value file's ASST SURG column holds the same value. The code is on the surgeon's bill and on the assistant's, so a patient can look it up without help. The indicator describes Medicare's position, and a commercial plan's list may differ, so the plan's own assistant-at-surgery policy is the second thing to ask for.

Then compare sizes. Divide the assistant's allowed amount by the surgeon's allowed amount, taken from the EOBs for the same date of surgery, since the two claims usually arrive separately. A result near 0.16 for a physician, or near 0.14 for a PA, NP or CNS, matches the Medicare-style rule and the UnitedHealthcare policy. If the surgeon's allowed amount is $3,000.00 and the assistant's is $480.00, the ratio is 0.16. If the assistant's allowed amount is far larger, the line is not following that rule, which is a question for the plan and not a conclusion: contracts can supersede policies, and some assistants are paid on negotiated rates.

Last, look at who owes what. When an in-network assistant's charge is larger than the allowed amount, the difference should appear on the remittance as a contracted adjustment, and the patient responsibility should be cost sharing on the allowed amount; the EOB shows both figures. When the assistant is out of network at an in-network facility, the No Surprises Act guide covers the rule. When a bill from an assistant arrives before any EOB, wait for the EOB rather than paying the first statement; the itemized bill checklist covers the other lines on the same day, and the guide on negotiating a hospital bill covers the conversation after the plan has processed everything.

For a planned operation, the best time to catch all of this is before the date of surgery. The pre-procedure billing guide has the list of lines to request; one added line for an assistant's identity, network status and expected modifier gets a written answer while there is still time to ask for a different arrangement, and the answer comes as a network status and an allowed amount instead of a statement received afterward.

Assistant surgeon claims: common questions

Primary sources for every rate and count

• CMS, Physician Fee Schedule relative value file RVU26D (October 2026 release; PPRRVU2026_Oct_nonQPP released 08/26/2026; GPCI2026; RVU26D documentation): RVUs, status codes, global days and the ASST SURG, CO-SURG and TEAM SURG indicators for 29881, 49505, 47562, 27130, 22612, 66984, 74177 and 90791, the indicator definitions, the $33.4009 conversion factor, and the indicator counts. Computed November 3, 2026.
• CMS, Medicare Claims Processing Manual, chapter 12 (Rev. 13753, issued 05-07-26): section 20.4.3 (assistant-at-surgery services), 40.1 (global surgical package), 40.8 (co-surgeons, team surgeons, and beneficiary liability on denied assistant claims), 100.1.7 (assistants at surgery in teaching hospitals), 110.2 and 120.1 (PA, NP and CNS assistants at surgery, 13.6 percent). Checked November 3, 2026.
• CMS, PFS Look-up Tool Overview: the tool's stated use for assistant-at-surgery payment policy. Checked November 3, 2026.
• U.S. Code, Title 42 (2024 edition, govinfo.gov): section 1395w-4(i)(2) (16 percent and the 5 percent test), section 1395l(a)(1)(O) (80 percent of 85 percent for PAs, NPs and CNSs, including assistants at surgery) and section 1395y(a)(15) (cataract assistants and the 5 percent exclusion). Checked November 3, 2026.
• eCFR, Title 42 (current to October 2026): 414.52(d) and 414.56(c) (85 percent for assistant-at-surgery services), 411.15(n) (excluded assistant services) and 415.190 (assistants at surgery in teaching hospitals). Checked November 3, 2026.
• X12, Claim Adjustment Reason Codes: code 54, "Multiple physicians/assistants are not covered in this case," and code 45, "Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement." Checked November 3, 2026.
• UnitedHealthcare, Commercial Reimbursement Policy 2026R5000D, Assistant-at-Surgery Services Policy, Professional: eligible list, 16 and 14 percent rates, one assistant, teaching hospitals, multiple procedures, surgical technicians. Checked November 3, 2026.

General billing information, not medical, legal or insurance advice. Medicare anchors are not your bill, a commercial plan's allowed amounts come from its own contracts, and a plan's policy can change after the date checked. Confirm your price in writing before service.