Medicare Coverage: Parts A, B, C, and D for US Patients

Medicare is the federal health insurance program covering roughly 65 million Americans — primarily adults 65 and older, along with younger individuals with certain disabilities or end-stage renal disease. The program is administered by the Centers for Medicare & Medicaid Services (CMS) and operates through four distinct coverage parts, each designed to address a different slice of the healthcare cost picture. Understanding which part covers what — and how the pieces interact — shapes real decisions about hospital stays, prescriptions, and monthly premiums.

Definition and Scope

Medicare was established under Title XVIII of the Social Security Act in 1965. As of 2024, CMS reports the program serves approximately 65.7 million beneficiaries, making it one of the largest health insurance programs in the world by enrollment.

The four parts function as distinct coverage domains:

1. Part A (Hospital Insurance) — Covers inpatient hospital care, skilled nursing facility stays following a qualifying hospital admission, hospice care, and limited home health services. Most people pay no monthly premium for Part A if they or their spouse worked and paid Medicare taxes for at least 40 quarters (10 years).

2. Part B (Medical Insurance) — Covers outpatient services: doctor visits, preventive screenings, durable medical equipment, and outpatient mental health care. The standard monthly premium for Part B in 2024 is $174.70, per CMS.

3. Part C (Medicare Advantage) — An alternative to Original Medicare (Parts A and B) offered through private insurers approved by CMS. Plans must cover at least everything Original Medicare covers and often bundle prescription drug coverage. Enrollment reached over 33 million beneficiaries by 2024, according to KFF.

4. Part D (Prescription Drug Coverage) — Standalone drug plans for those in Original Medicare, or integrated into most Medicare Advantage plans. Premiums and formularies vary by plan and region. The Medicare Prescription Payment Plan, a newer cost-smoothing feature under the Inflation Reduction Act, caps out-of-pocket drug costs at $2,000 annually beginning in 2025 (CMS).

The line between Medicare and Medicaid trips up a lot of people. Medicare is age- and disability-based, federally funded, and largely uniform across states. Medicaid is income-based, jointly funded by states and the federal government, and varies significantly by state.

How It Works

Original Medicare (Parts A + B) operates as a fee-for-service program: CMS pays providers directly for covered services after they're rendered. Beneficiaries share costs through deductibles, coinsurance, and copayments rather than a single monthly premium that covers everything.

For Part A, the inpatient hospital deductible in 2024 is $1,632 per benefit period — not per year (CMS). A "benefit period" resets 60 days after discharge, meaning someone with two separate hospitalizations in a year could pay that deductible twice.

Part B applies a 20% coinsurance after the annual deductible ($240 in 2024), and notably has no out-of-pocket maximum under Original Medicare alone. That gap is the reason Medigap supplemental insurance policies exist — they're sold by private insurers to cover cost-sharing that Original Medicare leaves exposed.

Medicare Advantage (Part C) flips the model: enrollees receive coverage through a private plan with network restrictions, prior authorization requirements, and typically a built-in annual out-of-pocket maximum. Understanding the full landscape of healthcare coverage options is useful context before choosing between these tracks.

Common Scenarios

Scenario: Hip replacement at 68. Part A covers the inpatient hospital stay. Part B covers the surgeon and anesthesiologist (as physician services billed separately). Post-discharge skilled nursing facility rehabilitation is covered by Part A only if the beneficiary had a qualifying hospital stay of at least 3 consecutive inpatient days — an administrative line that catches patients off-guard when observation stays don't count.

Scenario: Managing Type 2 diabetes long-term. Endocrinologist visits fall under Part B. Insulin, glucose monitors, and test strips may be covered under Part B (as durable medical equipment) or Part D depending on how they're dispensed. Chronic disease management often spans both parts, requiring beneficiaries to track which coverage applies to which supply.

Scenario: Mental health services. Part B covers outpatient mental health visits at the same 80/20 cost-sharing as medical care — parity that was strengthened by the Mental Health Parity and Addiction Equity Act. Inpatient psychiatric care is covered under Part A, though a 190-day lifetime limit applies to psychiatric facility stays, a restriction that doesn't apply to general hospital admissions.

Decision Boundaries

The central fork is Original Medicare versus Medicare Advantage — a choice that comes with real trade-offs, not just marketing differences.

Original Medicare allows access to virtually any provider who accepts Medicare — roughly 93% of non-pediatric physicians in the U.S., according to MedPAC's 2023 Report to Congress. It carries no network restrictions and no referral requirements. The downside: no cap on out-of-pocket exposure without supplemental coverage, and no bundled drug coverage without a standalone Part D plan.

Medicare Advantage typically offers lower premiums, out-of-pocket maximums, and extra benefits like dental and vision — categories where understanding health insurance trade-offs becomes genuinely consequential. The downside: narrow networks, prior authorization delays, and the possibility of a plan exiting a service area.

The enrollment windows are strict. The Initial Enrollment Period spans 7 months around a beneficiary's 65th birthday. Missing the Part B enrollment window without qualifying coverage triggers a 10% permanent premium penalty for each 12-month period of delay, per CMS. That penalty doesn't expire — it stays for as long as the beneficiary holds Part B coverage. For people approaching eligibility, the navigating the healthcare system framework applies in a concrete, calendar-driven way.