Medicare Part B infusion coinsurance: 20 percent of the drug price, with no yearly cap

Medicare.gov puts the rule in one line on its cost page: there is no yearly limit on what you pay out of pocket under Original Medicare unless you have supplemental coverage, such as a Medigap policy, or join a Medicare Advantage plan. Part D has a cap, $2,100 in 2026. Part B does not. For most Part B services that difference is small, because the bills are small. For infused biologic drugs, where one infusion can be billed in the thousands or tens of thousands of dollars, it is the largest number on the page.

How Medicare sets the price your 20 percent comes from

The coinsurance is a percentage of the Medicare-approved amount, so the approved amount is the number to understand first. For most drugs given in a doctor's office, the Social Security Act fixes it by formula. The statute, 42 USC 1395w-3a(b)(1), says the payment for a drug's billing code is, subject to the applicable deductible and coinsurance, 106 percent of the drug's average sales price, for single-source drugs and biologicals and for multiple-source drugs alike. That is the rule the industry calls ASP plus 6 percent.

Average sales price is a manufacturer-reported figure: sales to US purchasers in a quarter, net of most price concessions, divided by units sold. CMS turns it into a payment limit per HCPCS billing unit and publishes a new file every calendar quarter. The file now in force says on its first lines that it is effective October 1, 2026 through December 31, 2026 and that its limits are based on 2Q26 ASP data, which means the price a patient's 20 percent rests on this quarter reflects sales two quarters back. The 6 percent is already inside the published figure. Divide the J9271 limit of $62.253 by 1.06 and the implied average sales price is about $58.73 per milligram; the limit, not the ASP, is the Medicare-approved amount per unit.

Three refinements show up in the file's notes column. Qualifying biosimilars are paid at their own ASP plus 8 percent of the reference product's ASP, a temporary increase the statute sets for a five-year period, and the file labels those rows "8% of reference add-on applied". Some codes carry a note that a "lesser of" methodology applied, which happens when CMS compares ASP with another price measure and pays the lower; J0129, abatacept, is one this quarter. And a newly launched drug can be paid on wholesale acquisition cost before it has ASP history. None of those changes the 20 percent; each changes the number the 20 percent is taken from.

One protection sits on the other side of the price. Medicare.gov's page on Part B drugs says doctors, other providers and pharmacies must accept assignment for Part B-covered drugs, so they should never ask you to pay more than the coinsurance or copayment for the drug itself. The approved amount is a ceiling on the drug line: an office cannot add an excess charge above it the way a non-participating doctor can on some services.

October to December 2026 payment limits for six infusion codes

The table copies six rows from the October 2026 Medicare Part B Payment Limit File, the CMS file posted as final on September 17, 2026. They were chosen because they are high-volume infused or injected biologics that produce the large Part B drug lines readers ask about, and because together they show the three coinsurance situations in the file: the standard 20 percent, a biosimilar priced with the 8 percent add-on, and an inflation-adjusted percentage below 20. The same file holds the vaccine rows, where the coinsurance is 0.000; the guide to adult vaccine prices and the Medicare payment limit sets those limits beside the CDC price list.

Source: CMS, October 2026 Medicare Part B Payment Limit File, effective October 1, 2026 through December 31, 2026, based on 2Q26 ASP data. Checked 2026-10-30. National payment limits per billing unit, not patient prices. The share column applies after the Part B deductible.

Two codes in the table share a payment limit of $29.856, J1745 and J0897, although one is billed per 10 mg and the other per 1 mg. That is a coincidence of this quarter's figures, not an error in the copy; the file prints both values exactly. What separates them is the coinsurance column. J1745, the infliximab reference product, is at 20.000 percent. J0897, denosumab, is at 17.885%, because CMS marks it with inflation-adjusted coinsurance for this quarter. The biosimilar row, Q5103 (infliximab-dyyb, sold as Inflectra), is $28.456 per 10 mg against $29.856 for the reference product, and the file notes the 8 percent add-on.

From a per-milligram limit to a per-infusion line

The payment limit is per billing unit, and the unit is in the code's descriptor: 1 mg for J9271, J2350 and J0897, 10 mg for J1745, J9312 and Q5103. The claim records how many units were given, and the approved amount for the line is the limit times the units. A 200 mg dose of a drug billed per 1 mg is 200 units. A 500 mg dose of a drug billed per 10 mg is 50 units. When a single-dose container holds more than the dose and the rest is discarded, CMS's discarded-drug page says providers report the discarded amount on the claim with the JW modifier, or attest with the JZ modifier that nothing was discarded. A paid JW line is part of the approved amount, so the 20 percent applies to it too. The same unit rule runs through small injected drugs as well: the cortisone shot cost guide converts steroid doses into J3301, J1100 and J1010 units beside the joint injection line, and the eye injection cost guide does it for aflibercept, ranibizumab and faricimab, where the billing unit is 1 mg for one drug and 0.1 mg for another.

The illustrations below use stated inputs. The unit counts and the number of infusions a year are chosen to show the arithmetic; they are not dosing statements, and real regimens vary by label, weight, schedule and the prescriber's order, none of which this site comments on.

Payment limits from the CMS October 2026 Part B file (Q4 2026). Illustrations with stated inputs, computed by MyCarePrice and checked 2026-10-30. Drug line only; the administration line is separate and shown further down.

A year at 20 percent, set against the $2,100 Part D cap

The yearly figure is where Part B and Part D part ways. Run each illustration for a full year, holding the October 2026 limits constant for simplicity (in reality they reset every quarter, up or down), and the coinsurance on the drug line alone comes to the totals below. The last column puts the Part D out-of-pocket threshold next to them: $2,100 for 2026 according to Medicare.gov, rising to $2,400 in 2027 under the CMS CY2027 Rate Announcement.

Illustrations with stated inputs at Q4 2026 payment limits held for 12 months. Add $226.40 to any one row if the $283 Part B deductible is still unmet when the first claim of the year is processed. Drug line only.

Illustration A's coinsurance, $42,332.04 on $211,660.20 of approved drug charges, is more than twenty times the 2026 Part D cap. Illustration B, two infusions a year, is $14,565.36. Even illustration C, the smallest infusion line in the table, reaches $1,791.36 before any administration fee. The deductible adjustment is small by comparison: if a person's first Part B claim of 2026 is the infusion, the first $283 of the approved amount is paid in full instead of at 20 percent, which adds $226.40 once for the year.

The Part D cap does not help here for a structural reason, not a technical one. Medicare.gov explains that Part B usually covers drugs you would not typically give yourself, such as those given at a doctor's office or in a hospital outpatient setting, while Part D covers many drugs Part B does not. A drug that is infused in a clinic is billed by the clinic to Part B. Spending on it does not count toward the Part D threshold, and our Part D cap guide explains what does. The same split rules out the Medicare Prescription Payment Plan, which spreads Part D pharmacy costs across the year and has no role on a Part B claim.

The administration line: office versus hospital outpatient

The drug is one line. Putting it into a vein is another, billed with a drug-administration code, and the setting decides how that line is priced. In a physician office, the practice bills the administration code under the Physician Fee Schedule at the nonfacility rate. In a hospital outpatient department, the hospital bills it under the Outpatient Prospective Payment System. The CMS relative value file lists no facility rate for these administration codes, which is the file's way of saying the physician does not bill the administration when the hospital does.

Source: CMS PFS RVU26D (October 2026 release) and GPCI2026 for Michigan Locality 01 at the $33.4009 conversion factor, computed by MyCarePrice; CMS OPPS October 2026 Addendum B (file dated October 14, 2026), national unadjusted rates before wage-index adjustment. Checked 2026-10-30. Medicare anchors, not your bill; other localities and hospitals differ.

The setting changes one line and not the other. First, the drug line itself does not change with the setting for these codes. The OPPS October 2026 Addendum B lists J9271, J2350, J1745, J9312 and J0897 with status indicator K, separately paid, at exactly the per-unit rates in the Part B payment limit file: $62.253 for J9271, for example, with a per-unit copayment the hospital file prints rounded to the cent. Second, the administration line does change. Illustration A's 17 infusions with a one-hour chemotherapy administration under CPT 96413 add $444.04 in coinsurance in an office at the Michigan Locality 01 anchor, and $1,147.50 at the hospital's national unadjusted minimum copayment. Each additional hour adds an add-on code, 96415 or 96366. The OPPS column describes an on-campus hospital department: since January 1, 2026, the CY2026 OPPS rule pays drug administration in excepted off-campus departments at a lower, physician-fee-schedule-equivalent rate, so an off-campus hospital infusion suite can land between the two columns. The setting difference is the facility split described in the facility fee guide, and the Medicare outpatient procedure guide shows how the hospital copayment and the separate professional share stack on other outpatient services.

The hospital setting has one more rule that matters for big procedure bills: Addendum B notes that copayments for a single service are capped at the inpatient deductible, $1,736 in 2026, on the procedure rows where that cap binds. The drug rows in the same file carry a per-unit copayment and no cap note, so this guide does not assume the cap trims a large drug line. If a hospital-based infusion center quotes a figure, ask how the copayment on the drug line was computed and whether any cap was applied.

The inflation-rebate discount: 84 codes this quarter

The one reduction to the 20 percent that applies automatically comes from the Inflation Reduction Act. Under 42 USC 1395w-3a(i)(5), when a Part B rebatable drug's payment amount for a quarter exceeds its inflation-adjusted payment amount, the coinsurance is computed as 20 percent of the inflation-adjusted amount and then applied as a percentage of the normal payment. In plain terms: if a drug's price has risen faster than general inflation since its benchmark, the patient's share is figured as if it had not, and the manufacturer owes Medicare a rebate on the difference. CMS describes it the same way on its inflation rebates page, and says people with Medicare began seeing the lower coinsurance on April 1, 2023.

Where to find the current list has changed. CMS's page notes that, effective the second quarter of 2025, it no longer publishes a quarterly fact sheet listing the Part B rebatable drugs with adjusted coinsurance. The list now lives in the payment limit file itself, in the coinsurance percentage column, with the note "Inflation-adjusted coinsurance". In the October to December 2026 file, 84 of the 918 billing codes carry that note, with percentages from 0.997% to 19.970%. Most sit between 17 and 20 percent. A dozen are below 15 percent.

Ten of the 84 inflation-adjusted rows in the CMS October 2026 Medicare Part B Payment Limit File, effective October 1, 2026 through December 31, 2026. Checked 2026-10-30. The list resets every quarter.

Illustration D shows the size of the effect. Two 60-unit denosumab lines a year at $29.856 are $3,582.72 in approved charges. At a flat 20 percent the coinsurance would be $716.54; at this quarter's 17.885% it is $640.77, a difference of $75.77 for the year. The reduction is real but modest, because it trims the percentage by a point or three rather than capping the total. And because it is set quarter by quarter, a drug can be on the list for one infusion and off it for the next. Medicare.gov says as much: the specific drugs and potential savings change every quarter.

Closing the open-ended exposure with Medigap

A Medicare Supplement policy pays some or all of the Part B coinsurance after Original Medicare pays its 80 percent, which is the reason the uncapped 20 percent is mainly a concern for people with Original Medicare and nothing behind it. Medicare.gov's plan comparison chart lists how each standardized plan treats the two Part B cost-sharing rows that matter here.

Source: Medicare.gov, Compare Medigap plan benefits (2026 figures); CMS, K and L Out-of-Pocket Limits, October 2026 announcement (2027 figures). Checked 2026-10-30. Premiums vary by insurer, state and plan and are not shown.

Run illustration A through the chart. Under Plan G, the policy pays the $42,332.04 coinsurance, and the person's cost sharing for the drug line is the $283 Part B deductible, plus the monthly premiums for Part B and the policy. Under Plan K, the policy pays half the coinsurance until the person's own spending reaches the yearly limit; half of $42,332.04 is far above $8,000, so the person's share stops at about $8,000 for 2026, after which Medicare.gov says Plan K pays 100 percent of covered services for the rest of the calendar year. Plan L stops at $4,000. How the Part B deductible sits against the K and L limits is described on Medicare.gov as meeting both the yearly limit and the Part B deductible; ask the insurer to confirm the order on its own policy.

Timing is the catch. A Medigap policy pays only on claims for services furnished while it is in force, and the right to buy one on standard terms depends on when you apply. Those enrollment rules are outside this site's price focus; the Medigap pages on Medicare.gov describe them. The cost point is narrower: a person facing a year of high-cost infusions under Original Medicare alone has no ceiling, and the plan type decides whether the ceiling is zero, $4,000, $8,000 or absent.

Closing it with a Medicare Advantage out-of-pocket maximum

Medicare Advantage plans must set a yearly maximum out-of-pocket amount for in-network Part A and Part B services, and Part B drugs count toward it. Medicare.gov's cost page says that once you pay the plan's limit, the plan pays 100 percent of covered health services for the rest of the calendar year. The plan sets its own limit, but CMS caps how high it can go, and the CMS MOOP calculation files give the ceilings.

Source: CMS, CY 2026 and CY 2027 MOOP and Cost Sharing Limit Calculations, Medicare Advantage Rates and Statistics page. Checked 2026-10-30. Plans may set lower limits. Coinsurance for Part B drugs in a plan is capped at 20 percent under 42 CFR 422.100(j)(1), as the same files state.

Under the 2026 ceiling, illustration A's $42,332.04 in drug coinsurance would stop at no more than $9,250 at an in-network infusion site, and less in a plan with a lower MOOP. The copayment limits in the table are how CMS turns the 20 percent ceiling into a flat-dollar equivalent: for 2027 it projects a median Medicare allowed amount per visit for Part B drugs and takes 20 percent of it, which gives the $335 and $450 figures. A plan can charge coinsurance of up to 20 percent instead of a flat copayment, and on a large drug line that coinsurance is what drives spending to the MOOP quickly.

The trade-offs are the network and the approval process. The MOOP that applies to an out-of-network infusion center is the combined figure, or none at all in an HMO that does not cover out-of-network care. A plan can require prior authorization for a Part B drug, and a denied claim does not count toward the MOOP unless the denial is reversed. The prior authorization guide covers what an approval number proves, and deductible versus out-of-pocket maximum explains how a plan counts spending toward its limit.

Who else can sit behind the 20 percent

Medigap and Medicare Advantage are the two routes Medicare.gov names, but a Part B claim can have other payers behind it. Employer retiree coverage or a current employer plan can also pay on a Part B claim, in an order set by coordination of benefits rules that the plan administrator can explain. Medicare.gov points people with limited income and resources to help with Part A and Part B costs; our Medicare Savings Programs guide sets out which programs help with Part B cost sharing and the limits that decide eligibility. An infusion center run by a hospital may also fall under the hospital's financial assistance policy, which the charity care guide explains how to request.

Each of those has its own rules and paperwork, and this guide does not price them. The number to bring to any of them is the same: the HCPCS code, the units per infusion, the quarter's payment limit, and the expected count of infusions for the year, which together give the approved amount and the 20 percent. Drugs given during an outpatient dialysis session work differently: they sit inside a bundled per-treatment payment, and the guide to dialysis cost per treatment with Medicare shows how the 20 percent is figured there.

Checking a Part B drug line on the Medicare Summary Notice

Part B drug lines are among the easiest to check, because each one is a code, a unit count and a published per-unit limit. Take the Medicare Summary Notice or the explanation of benefits from your Medicare Advantage plan, and for each drug line confirm four things.

The quarter matters more on drug lines than anywhere else on a bill. An infusion on September 30 is priced from the July 2026 file and one on October 1 from the October file, and the two can differ in both the limit and the coinsurance percentage. When a line looks wrong, ask the billing office which quarter's limit it used. How to read an explanation of benefits shows where the approved amount and your share appear, and the itemized-bill check covers the request for line-level detail.

Because Part B drug providers must accept assignment, an office that bills you more than the coinsurance on the drug line, after Medicare has processed the claim, has a question to answer. Put it in writing with the code, the units, the date of service and the amount from the notice.

Part B drug coinsurance: reader questions

Where the ASP limits and the Part D cap come from

• Office of the Law Revision Counsel, 42 USC 1395w-3a, Use of average sales price payment methodology: subsection (b)(1) (106 percent of ASP), (b)(8) (biosimilar add-on of 8 percent during the applicable five-year period) and (i)(5) (coinsurance for Part B rebatable drugs). Code current through Public Law 119-136. Checked October 30, 2026.
• CMS, Discarded Drugs (JW and JZ modifiers). Checked October 30, 2026.
• CMS, ASP Pricing Files, and the October 2026 Medicare Part B Payment Limit File (effective October 1, 2026 through December 31, 2026, based on 2Q26 ASP data, final file posted September 17, 2026): every payment limit, coinsurance percentage and inflation-adjusted row on this page. Checked October 30, 2026.
• CMS, Inflation Rebates in Medicare: coinsurance adjustment for Part B rebatable drugs since April 1, 2023, and the end of quarterly fact sheets from the second quarter of 2025. Checked October 30, 2026.
• CMS, PFS relative value file RVU26D (October 2026 release) and CY2026 GPCIs, Michigan Locality 01, $33.4009 conversion factor, for the 96365, 96366, 96413 and 96415 office anchors; and the OPPS October 2026 Addendum B for hospital rates, drug status indicators and copayments. Computed and checked October 30, 2026.
• Medicare.gov, Medicare costs (2026 Part B deductible of $283, no yearly out-of-pocket limit in Original Medicare, 2026 Part D limit of $2,100) and Prescription drugs (outpatient) (Part B versus Part D, mandatory assignment, quarterly coinsurance changes). Checked October 30, 2026.
• CMS, Announcement of CY 2027 Medicare Advantage Capitation Rates and Part C and Part D Payment Policies: 2027 Part D out-of-pocket threshold of $2,400. Checked October 30, 2026.
• CMS, Medicare Advantage Rates and Statistics, CY 2026 and CY 2027 MOOP and Cost Sharing Limit Calculations: in-network and combined MOOP limits and Part B drug copayment limits. Checked October 30, 2026.
• Medicare.gov, Compare Medigap plan benefits, and CMS, K and L Out-of-Pocket Limits Announcements (October 2026 announcement for 2027). Checked October 30, 2026.

General billing information, not medical, legal or insurance advice. Medicare payment limits and fee schedule amounts are anchors, not your bill, and a plan's own cost sharing can differ. Confirm your price in writing before service.