Original Medicare’s Part B benefit runs on a formula that catches many enrollees off guard the first time a real bill arrives. Once a covered service has been approved and the annual deductible has been met, the program pays 80 percent of the Medicare-approved amount, and the patient owes the remaining 20 percent. That split is written into the statute governing Medicare, it applies across nearly every outpatient service Part B covers, and it comes with no yearly ceiling that stops the 20 percent share from adding up as care continues.
The 80/20 Split Behind Every Approved Bill
The math applies only after the Part B deductible has been satisfied for the year; from that point forward, Medicare pays its 80 percent share on each Medicare-approved service. A primary care visit, an outpatient procedure, physical therapy sessions, durable medical equipment and most lab work all fall under the same coinsurance structure, according to Medicare’s own breakdown of Part B costs. The word “approved” is the operative one: Medicare sets a maximum allowable charge for each service, and the 80/20 split is calculated against that approved figure, not against whatever a provider’s standard rate might otherwise be. That distinction protects patients from a provider simply inflating a bill to raise its own share, since the 80 percent payment and the 20 percent coinsurance are both pegged to the same government-set number regardless of what the provider originally charged.
Because coinsurance is a percentage rather than a flat copay, the size of the patient’s 20 percent share rises and falls with the size of the underlying bill. A brief office visit produces a small coinsurance charge, while a costly outpatient surgery produces a proportionally larger one, and no single-service cap limits how high that percentage-based bill can climb. A person managing a chronic condition that requires frequent specialist visits, imaging or ongoing infusion therapy can accumulate a sizable coinsurance total over a year through nothing more than routine, medically necessary care. Each new approved service simply restarts the same 80/20 calculation, so a patient juggling several conditions at once can be tracking coinsurance owed to more than one provider in the same month.
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Original Medicare Was Never Built With a Stopping Point
Unlike an employer health plan or a Medicare Advantage plan, Original Medicare has no built-in annual out-of-pocket maximum. Federal rules require every Medicare Advantage plan to cap a member’s yearly spending on covered services, but that requirement does not extend to Original Medicare, so a Part B enrollee’s 20 percent coinsurance can keep accumulating for as long as covered care continues during the year. Someone hospitalized for a serious illness who also needs extensive outpatient follow-up, physical therapy and specialist care afterward can end up owing 20 percent coinsurance on each of those services in succession, with nothing in Original Medicare’s design to stop the total once it starts climbing.
That open-ended exposure is exactly why many enrollees pair Original Medicare with a private Medigap policy, which exists specifically to pick up some or all of the 20 percent that Part B leaves on the table. Enrollees with limited income and resources have a separate route: state-administered Medicare Savings Programs can cover coinsurance entirely for those who qualify, an option Medicare’s own guidance on savings programs spells out by state.
Assignment Decides Whether the 20 Percent Grows Further
The 80/20 split assumes a provider has agreed to what Medicare calls “accepting assignment,” meaning the provider takes the Medicare-approved amount as full payment for a covered service. A provider who accepts assignment can only bill the deductible and the 20 percent coinsurance, and must submit the claim to Medicare directly rather than asking the patient to pay the full cost upfront, according to Medicare’s explanation of provider assignment.
A “non-participating” provider, one who has not agreed to accept assignment on every claim, can still charge more than the Medicare-approved amount, though federal law caps that markup, known as the limiting charge, at 15 percent above the approved rate in most cases. That 15 percent ceiling is itself fixed by statute and applies on top of the standard 20 percent coinsurance, meaning a patient seeing a non-participating provider can end up owing more than the usual coinsurance share even though Medicare’s underlying 80/20 split has not changed.
Managing a Share That Has No Ceiling
For enrollees who have not purchased a Medigap policy and do not qualify for Medicaid or a Medicare Savings Program, the coinsurance obligation stays theirs to track bill by bill for as long as they keep receiving Part B services. Medicare directs enrollees who are struggling with a running coinsurance total to their local State Health Insurance Assistance Program, which offers free, unbiased counseling on Medigap options, Medicare Savings Programs and other ways to close the 20 percent gap before it grows into an unmanageable balance.
The gap also explains why so much of the Medicare marketing an enrollee sees each fall focuses on plans built around that missing ceiling rather than on the 80/20 split itself. A Medicare Advantage plan’s federally required out-of-pocket limit and a Medigap policy’s coinsurance coverage are two different answers to the same underlying feature of Original Medicare: the program will keep paying 80 percent indefinitely, but it was never designed to promise that the remaining 20 percent stays affordable on its own.
This article was produced with AI assistance and reviewed by The Financial Wire editorial team.
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David M. Keller is a finance writer based in Columbus, Ohio, covering personal finance and consumer-focused economic topics. He earned his degree in journalism from Ohio University and began his career reporting on local business and economic trends for a regional media outlet. Since then, he has contributed to a variety of online publications, focusing on clear, practical coverage of topics such as cost of living, debt, and everyday financial decision-making.
