Medicare Part B

Medicare Part B is the component of Medicare that covers medical and outpatient services. Congress named it Medical Insurance, and it is established by part B of title XVIII of the Social Security Act, codified at 42 U.S.C. 1395j and the sections following. Medicare's own consumer materials label it "Part B (Medical Insurance)", parallel to "Part A (Hospital Insurance)" and "Part D (Drug coverage)".

Medicare.gov describes what it helps cover as "Services from doctors and other health care providers; Outpatient care; Home health care; Durable medical equipment (like wheelchairs, walkers, hospital beds, and other equipment); Many preventive services (like screenings, shots or vaccines, and yearly 'Wellness' visits)". The division of labor with Part A is worth stating once, because it surprises people: Part A pays the facility for an inpatient stay, and Part B pays the physicians who treat you during it. A hospital admission therefore generates bills under both parts.

🔑 The most consequential fact about Part B is what it does not have. Medicare.gov states it without qualification: "There's no yearly limit on what you pay out-of-pocket, unless you have supplemental coverage, like a Medicare Supplement Insurance (Medigap) policy, or you join a Medicare Advantage Plan." Anyone arriving at 65 from an employer plan has spent decades with an out-of-pocket maximum and is likely to assume one exists here. It does not. Combined with the flat coinsurance below, that absence is the reason the supplement decision is effectively mandatory even though it is legally optional.

The cost structure, in the parts that do not change from year to year. The dollar amounts are reset every autumn, but the shape is stable and knowing the shape is more useful than memorizing a figure. There is one annual deductible, and medicare.gov is explicit that "You pay this deductible once each year", which is the cleanest available contrast with Part A's per-benefit-period deductible. Above it, the enrollee usually pays 20% of the Medicare-approved amount for each covered service or item. Several categories sit outside that 20%: medicare.gov puts covered clinical laboratory services and covered home health services at nothing, and states that you pay nothing for most preventive services provided the clinician accepts assignment. Care received in a hospital outpatient department carries an additional copayment to the hospital, which medicare.gov says will in most cases not exceed the Part A inpatient hospital deductible. That is a ratio rather than a figure, and it is the reason the same procedure can cost more in a hospital outpatient department than in a physician's office.

Assignment decides how much of the bill Medicare's approved amount actually controls, and there are three provider positions rather than two. A provider who accepts assignment agrees to take the Medicare-approved amount as payment in full, must bill Medicare directly, and may charge you only the deductible and coinsurance. Medicare.gov notes that where a provider accepts assignment, it applies to all Medicare-covered Part A and Part B services. A non-participating provider has not agreed in advance but may accept the approved amount case by case; where they do not, medicare.gov says that "In many cases, the charge can't be more than 15% above the Medicare-approved amount", an excess known as the limiting charge, and you may have to pay the full amount at the time of service and claim it back. A provider who has opted out of Medicare is a different situation altogether: Medicare will not pay for items or services from them except in emergencies, the arrangement runs through a private contract, and the opt-out lasts a minimum of two years. Checking which of the three applies before a course of expensive treatment is one of the few genuinely actionable things in this area.

The premium is where Part B differs most from Part A. Part A is premium-free for the large majority of beneficiaries because it was prepaid through payroll tax. Part B is not: it carries a monthly premium that, medicare.gov notes, is owed "even if you don't get any Part B-covered services", and the premium rises with income above defined thresholds. The income-related surcharge has its own name and its own mechanics, including a two-year lookback to a prior tax return, and it belongs to its own entry rather than here. Enrollees with limited income and resources may have the premium paid by their state.

Enrollment is automatic for some people and not others, and getting this backwards is expensive. Anyone already drawing Social Security or Railroad Retirement benefits at 65 is enrolled in Parts A and B automatically and mailed a card. Someone who reaches 65 while deliberately delaying a Social Security claim has to sign up, and it is that person who is exposed to the enrollment deadline and to a late-enrollment penalty that attaches to the Part B premium for as long as they hold Part B. The windows and the penalty arithmetic belong to their own entries; what belongs here is that the penalty is permanent rather than a one-off.

⚠️ One further piece of Medicare vocabulary reverses a common assumption. "Primary" and "secondary" in this context are Medicare Secondary Payer terms about which insurer pays first, and Medicare is frequently not the primary payer at 65. Where someone has group coverage through current employment at an employer with 20 or more employees, the group plan pays first and Medicare pays second. Below that size threshold the order reverses. This matters most to people still working past 65, which is exactly the population most likely to be told that Medicare "becomes their coverage" at 65.

Two structural dependencies round out the picture. Part B enrollment is a condition of joining a Medicare Advantage plan, and the Part B premium has to keep being paid to stay in one. It is also a condition of buying into Part A for the small group without the work record for premium-free Part A, and dropping Part B ends that bought-in Part A coverage at the same time.

1. The annual deductible is charged once per calendar year, on the first covered Part B services of that year.

2. Coinsurance applies above it, usually 20% of the Medicare-approved amount for each covered service or item, with several categories such as covered clinical laboratory services, covered home health services and most preventive care carrying nothing.

3. Assignment decides the base. A provider accepting assignment takes the approved amount as full payment. A non-participating provider may add a limiting charge on top. A provider who has opted out is paid by private contract and Medicare pays nothing except in an emergency.

4. Nothing stops the coinsurance. There is no annual ceiling on the enrollee's share, so the 20% continues for as long as covered care does.

5. The gap is filled privately or not at all, through a Medigap policy alongside Original Medicare or by joining a Medicare Advantage plan, which must cap in-network spending.

6. The deductible resets on January 1, and the count begins again. A hypothetical, using illustrative approved amounts to show the missing ceiling. Assume the year's Part B deductible has already been met and a course of covered outpatient treatment carries Medicare-approved amounts totalling $40,000. Part B pays 80%, or $32,000, and the enrollee's 20% share is $8,000. Now suppose the treatment runs longer and the approved amounts reach $80,000: Part B pays $64,000 and the enrollee's share doubles to $16,000. There is no point in that sequence at which the share stops growing, because the percentage is fixed and nothing caps the total. An employer plan or a Marketplace plan with a $7,000 out-of-pocket maximum would have stopped the enrollee's share at $7,000 in both cases, because both $8,000 and $16,000 exceed it. The approved amounts here are illustrative; the absence of a ceiling is not.

A second point the arithmetic makes quietly. Because the share is a percentage of the approved amount rather than a fixed sum, the enrollee's exposure scales with the severity of the illness. That is the opposite of how most modern health coverage behaves, and it is the specific risk a Medigap policy is bought to remove.

Pros

• Coverage is broad, portable and accepted by most clinicians in the country, without a network in the sense an employer plan or a Medicare Advantage plan uses the word.
• Most preventive services carry no cost sharing at all where the provider accepts assignment, and covered clinical laboratory and home health services generally carry none either.
• Entitlement does not depend on health. There is no underwriting and no medical questionnaire.
• The deductible is annual and modest relative to the care it unlocks, and it is charged only once however many separate episodes of care occur.
• Assignment gives real price protection. Where a provider accepts it, the Medicare-approved amount is the whole bill.

Cons

• There is no out-of-pocket maximum, so a serious illness has no financial stopping point without supplemental coverage.
• The 20% coinsurance is a percentage rather than a fixed amount, so exposure grows with the size of the bill rather than being capped by it.
• It carries a monthly premium for essentially everyone, owed whether or not any services are used, and the premium rises with income.
• Someone not yet drawing Social Security has to enrol actively, and a late-enrollment penalty attaches to the premium for as long as they hold Part B rather than for a fixed period.
• A provider who has opted out of Medicare is paid entirely by private contract, and Medicare pays nothing toward that care except in an emergency.
• It covers nothing for routine dental, vision or hearing care, and no part of Medicare covers long-term custodial care.