Medicare Part A, B, C, and D: What Each Covers

Medicare is not a single insurance program. It is a federal health coverage framework divided into four distinct parts, each with its own scope of coverage, premium structure, and enrollment mechanics. Parts A and B form Original Medicare, administered directly by the federal government. Parts C and D layer on top of that foundation through contracts with private insurers operating under federal rules.

Understanding the structure requires treating each part as a separate program with its own triggers, costs, and gaps — not as interchangeable components of a unified benefit. The coverage boundaries between parts are precise, and the gaps between them are real. What one part pays for, another explicitly excludes.

How Each of the Four Parts Is Defined and What It Covers

Part A — Hospital Insurance. Part A covers inpatient hospital stays, skilled nursing facility care following a qualifying hospital stay, hospice care, and some home health services. Eligibility for premium-free Part A is tied to work history: a person (or their spouse) who paid Medicare taxes for at least 40 quarters — ten years — generally pays no monthly premium for Part A. Those with fewer than 40 quarters pay a premium that scales with the number of quarters worked, as set annually by the Centers for Medicare & Medicaid Services. Part A does not cover custodial or long-term nursing home care; it covers only skilled care following an acute event, subject to strict day limits and cost-sharing rules. The mechanics of how Part A hospital coverage applies — including benefit periods and the inpatient deductible — operate independently from the other parts.

Part B — Medical Insurance. Part B covers outpatient services: physician visits, preventive care, durable medical equipment, outpatient surgery, lab tests, and certain home health services not covered under Part A. Unlike Part A, Part B always carries a monthly premium. That premium is income-adjusted through a mechanism called the Income-Related Monthly Adjustment Amount (IRMAA), which applies to beneficiaries whose modified adjusted gross income from two years prior exceeds statutory thresholds. The way Part B premiums scale with income means higher earners pay a larger share of the program's actual cost. The standard Part B premium and IRMAA brackets are recalculated each year. Part B also carries an annual deductible and a 20 percent coinsurance requirement for most covered services, with no out-of-pocket maximum under Original Medicare alone.

Part C — Medicare Advantage. Part C, known as Medicare Advantage, is not a separate pool of coverage. It is a delivery mechanism: a beneficiary enrolled in both Part A and Part B can elect to receive all of that coverage through a private health plan that has contracted with the federal government. Medicare Advantage plans must cover everything Original Medicare covers, but they may also add benefits — such as dental, vision, or hearing — that Original Medicare does not include. They impose their own network restrictions, cost-sharing structures, and prior authorization requirements. The federal government pays the contracting insurer a risk-adjusted capitated amount per enrollee. Enrollment in Part C does not eliminate the Part B premium; the beneficiary continues to pay it.

Part D — Prescription Drug Coverage. Part D is a voluntary outpatient prescription drug benefit delivered through private insurers that contract with the federal government. Each Part D plan maintains a formulary — a list of covered drugs organized into cost-sharing tiers. Premiums, deductibles, and cost-sharing vary by plan. Part D also carries an IRMAA surcharge for higher-income beneficiaries, calculated separately from the Part B IRMAA. Beneficiaries who delay Part D enrollment past their initial eligibility window without creditable drug coverage from another source incur a permanent late-enrollment penalty added to their monthly premium.

Who Administers Each Part and Through What Structure

The Centers for Medicare & Medicaid Services (CMS), within the U.S. Department of Health and Human Services, sets the rules, benefit standards, and payment rates for all four parts. CMS does not process most claims directly; it contracts with private administrative entities called Medicare Administrative Contractors (MACs) to handle claims processing and provider payments for Parts A and B.

For Parts C and D, the administrative layer is more visible to the beneficiary. Private insurers — referred to in the regulatory framework as Medicare Advantage Organizations (for Part C) and Part D plan sponsors — contract with CMS under annual agreements. These organizations set their own premiums (subject to federal approval), design their own plan structures within CMS rules, and operate their own member service functions. CMS audits these plans, enforces compliance, and can terminate contracts. The Social Security Administration handles enrollment transactions and collects Part B and Part D premiums for most beneficiaries, typically by deducting them from monthly Social Security benefit payments.

Employers and unions may also sponsor Medicare coverage through arrangements called Employer Group Waiver Plans (EGWPs), which operate under the Medicare Advantage or Part D framework but are offered as a retiree benefit rather than through the public marketplace.

Where the Structure Produces Unexpected or Misunderstood Results

No out-of-pocket maximum in Original Medicare. Parts A and B together impose no statutory cap on total out-of-pocket costs in a given year. The 20 percent Part B coinsurance applies without limit. This is one of the most consequential gaps in the Original Medicare structure and is frequently misunderstood by beneficiaries who assume federal health coverage operates like employer-sponsored insurance.

The late-enrollment penalty for Part B is permanent. A beneficiary who does not enroll in Part B during the Initial Enrollment Period — and who does not qualify for a Special Enrollment Period based on current employer coverage — faces a 10 percent premium surcharge for each full 12-month period of delayed enrollment. That surcharge does not expire; it remains for as long as the beneficiary is enrolled in Part B. The same permanent-penalty structure applies to Part D, calculated at 1 percent of the national base beneficiary premium per month of uncovered delay.

Part C enrollment does not replace the Part B premium obligation. Beneficiaries who switch to Medicare Advantage sometimes expect the Part B premium to disappear. It does not. The Part B premium continues regardless of whether coverage is received through Original Medicare or through a Medicare Advantage plan. Some Part C plans offer a "Part B premium reduction" as a plan benefit, but this is a plan-level subsidy, not an elimination of the underlying premium obligation.

Coordination with employer coverage before Medicare eligibility. Individuals who retire before age 65 face a coverage gap. The interaction between continuation coverage and Medicare timing — including how COBRA coverage bridges the period before Medicare eligibility — involves distinct enrollment rules that do not automatically synchronize. Delayed Medicare enrollment in this window can trigger the permanent penalties described above if not managed precisely.

Part D formularies change annually. A drug covered on a plan's formulary in one year may be removed or moved to a higher cost-sharing tier in the next. The annual open enrollment period exists partly to allow beneficiaries to compare formularies as they change, but the formulary is not guaranteed to remain static during a plan year either; mid-year changes are permitted under specific CMS rules.

What Medicare Notices and Statements Show — and What They Omit

Beneficiaries enrolled in Original Medicare receive a Medicare Summary Notice (MSN) — historically a paper document, now also available electronically through the Medicare.gov portal — that itemizes claims processed under Parts A and B. The MSN shows the service date, the amount billed by the provider, the amount Medicare approved, the amount Medicare paid, and the amount the beneficiary may owe. It does not show cumulative out-of-pocket totals for the year, and it does not reflect any supplemental coverage that may have paid the remaining cost-sharing.

Part C enrollees receive an Explanation of Benefits (EOB) from their Medicare Advantage plan rather than an MSN from CMS. The EOB format and detail level vary by plan. CMS requires that EOBs be issued monthly when claims have been processed, but the content standards are less uniform than those for Original Medicare's MSN.

Part D enrollees receive an EOB from their Part D plan sponsor each month in which a prescription is filled. This document shows the drugs dispensed, the cost-sharing paid, and the cumulative amounts applied toward the deductible and toward the out-of-pocket threshold that governs catastrophic coverage. The Part D EOB is the primary mechanism through which a beneficiary can track progress through the benefit phases, but it reflects only claims processed by that specific plan — it does not aggregate costs across multiple Part D plans if a beneficiary changed plans during the year.

The Social Security Administration separately sends an annual notice — typically in November — showing the Part B and Part D premiums that will apply in the coming year, including any IRMAA determination. This notice reflects the income data CMS received from the IRS and is separate from any Medicare-issued document.

Medicare's four-part structure reflects decades of legislative layering rather than a unified design, which is why the coverage boundaries, premium rules, and enrollment windows operate on separate tracks that do not always align with intuitive expectations about how a single health program would work.

Note: This explains how a retirement system works. It is not financial, tax, or legal advice, it is not specific to any individual's retirement, and it is not a substitute for a licensed financial, tax, or legal professional. Rules, ages, and dollar limits change by year — check the cited sources.