Medicare Coverage for Senior Health Services: Parts A, B, C, and D Explained

Medicare is the federal health insurance program administered by the Centers for Medicare & Medicaid Services (CMS) that covers approximately 65 million Americans, primarily adults aged 65 and older (CMS, Medicare Program Statistics). The program is structured across four distinct parts — A, B, C, and D — each governing a separate category of covered services, cost-sharing rules, and enrollment mechanics. Understanding how these parts interact is essential for navigating senior primary care services, prescription coverage, and the full continuum of home health care services for seniors. This page provides a reference-grade breakdown of each part's scope, eligibility rules, coverage boundaries, and known tensions.

• Definition and scope
• Core mechanics or structure
• Causal relationships or drivers
• Classification boundaries
• Tradeoffs and tensions
• Common misconceptions
• Checklist or steps (non-advisory)
• Reference table or matrix

Definition and scope

Medicare is established under Title XVIII of the Social Security Act (42 U.S.C. §§ 1395–1395lll) and funded through a combination of payroll taxes collected under the Federal Insurance Contributions Act (FICA), general federal revenues, and beneficiary premiums. The Social Security Administration (SSA) handles initial eligibility determinations, while CMS administers ongoing coverage rules, contractor oversight, and the Medicare & Medicaid program boundaries.

Eligibility for standard Part A and Part B coverage applies to individuals who are:

• Age 65 or older and a U.S. citizen or permanent legal resident with at least 5 years of continuous residence
• Under 65 with a qualifying disability and 24 months of Social Security Disability Insurance (SSDI) receipt
• Diagnosed with End-Stage Renal Disease (ESRD) or Amyotrophic Lateral Sclerosis (ALS), which carry different waiting-period rules under CMS guidelines

The program does not constitute comprehensive insurance. Dental, vision, and hearing coverage — areas relevant to senior vision and eye care, senior hearing care services, and senior dental care services — are largely excluded from Original Medicare (Parts A and B), a gap that Part C (Medicare Advantage) plans may or may not address depending on plan design.

Core mechanics or structure

Part A: Hospital Insurance

Part A covers inpatient hospital stays, skilled nursing facility (SNF) care following a qualifying hospital stay of at least 3 consecutive inpatient days, hospice care, and limited home health services. Most beneficiaries pay no Part A premium if they or their spouse paid Medicare taxes for 40 or more quarters (10 years) of employment (CMS, Medicare Costs at a Glance).

Cost-sharing under Part A (2024 parameters per CMS):
- Inpatient hospital deductible: $1,632 per benefit period
- Days 1–60: $0 coinsurance per day
- Days 61–90: $408 coinsurance per day
- Days 91 and beyond (lifetime reserve days): $816 per day

Skilled nursing facility coverage follows a separate schedule: days 1–20 carry no coinsurance; days 21–100 carry $204 per day coinsurance (2024 CMS figures); beyond day 100, Medicare provides no coverage.

Part B: Medical Insurance

Part B covers outpatient physician services, preventive care, durable medical equipment (DME), mental health outpatient services, and certain home health services not requiring a preceding hospital stay. The standard Part B premium for 2024 is $174.70 per month (CMS, 2024 Medicare Parts A & B Premiums and Deductibles), though higher-income beneficiaries pay Income-Related Monthly Adjustment Amounts (IRMAA) set by SSA.

Part B cost-sharing:
- Annual deductible: $240 (2024)
- After deductible: 20% coinsurance for most covered services
- No out-of-pocket maximum in Original Medicare

Part B covers an annual wellness visit for seniors at no cost-sharing once per year, along with a separate "Welcome to Medicare" preventive visit during the first 12 months of Part B enrollment.

Part C: Medicare Advantage

Part C, governed under 42 C.F.R. Part 422, allows private insurers approved by CMS to deliver all Part A and Part B benefits through managed care plans — typically Health Maintenance Organizations (HMOs) or Preferred Provider Organizations (PPOs). Plans may include supplemental benefits such as dental, vision, hearing, or fitness programs not available under Original Medicare. Enrollment in Medicare Advantage reached 33.8 million beneficiaries in 2024 (KFF, Medicare Advantage in 2024).

Plans must cover all medically necessary services that Original Medicare covers, but may require in-network providers, referrals, and prior authorization for specialist visits.

Part D: Prescription Drug Coverage

Part D, authorized under the Medicare Prescription Drug, Improvement, and Modernization Act of 2003 (MMA, P.L. 108-173), provides outpatient prescription drug coverage through private plan sponsors under 42 C.F.R. Part 423. Plans use formularies — tiered drug lists — to determine coverage levels. The Inflation Reduction Act of 2022 (P.L. 117-169) restructured Part D cost-sharing effective 2025, capping out-of-pocket drug costs at $2,000 annually and eliminating the coverage gap ("donut hole") phase. Senior medication management is directly governed by Part D formulary rules and low-income subsidy (LIS) eligibility criteria.

Causal relationships or drivers

The four-part structure reflects legislative layering over six decades. Part A and B were established by the Social Security Amendments of 1965 (P.L. 89-97). Part C (then called Medicare+Choice) was created by the Balanced Budget Act of 1997 (P.L. 105-33) and renamed Medicare Advantage under the 2003 MMA. Part D was introduced by the same 2003 legislation.

Cost-sharing amounts are recalculated annually by CMS based on actuarial projections of program costs, beneficiary income distributions, and Congressional Budget Office (CBO) baseline estimates. The absence of a Part B out-of-pocket maximum is a structural feature — not an oversight — that reflects the original program design, which assumed beneficiaries would obtain Medigap (Medicare Supplement Insurance) coverage under standardized plans regulated by CMS and state insurance commissioners.

Premium surcharges under IRMAA are triggered when modified adjusted gross income (MAGI) reported to the Internal Revenue Service (IRS) in the prior tax year exceeds thresholds set annually by SSA.

Classification boundaries

Medicare does not cover all services a senior may require. CMS publishes National Coverage Determinations (NCDs) and Local Coverage Determinations (LCDs) — administered through Medicare Administrative Contractors (MACs) — to define covered and non-covered items. The boundary between "skilled" and "custodial" care is central to Part A SNF and home health eligibility:

• Skilled care: Requires the professional judgment of a licensed nurse or therapist and is medically necessary. Covered under Part A (SNF) or Part B (home health).
• Custodial care: Assistance with activities of daily living (bathing, dressing, eating) that does not require skilled professional oversight. Not covered by Original Medicare.

This distinction directly affects whether home health care services for seniors and senior rehabilitation services are reimbursable. The 1988 Jimmo v. Sebelius settlement (D. Vt.) clarified that Medicare coverage of skilled care is not contingent on a beneficiary's potential for improvement — the "improvement standard" — but on the medical necessity of skilled maintenance care.

Hospice benefits under Part A require a physician certification of terminal illness with a prognosis of 6 months or fewer if the disease runs its normal course, and beneficiaries must elect to forgo curative treatment for the terminal diagnosis. Hospice and palliative care for seniors operates under this distinct election framework.

Tradeoffs and tensions

Original Medicare vs. Medicare Advantage: Original Medicare provides uniform national coverage with unrestricted provider choice among participating providers. Medicare Advantage imposes network restrictions, referral requirements, and prior authorization protocols but may offer lower cost-sharing and supplemental benefits. KFF analysis of 2024 data indicates that 57% of Medicare-eligible beneficiaries are enrolled in Medicare Advantage plans, raising policy questions about prior authorization denials and network adequacy in rural areas — issues tracked by the HHS Office of Inspector General (OIG).

Medigap and Part D interaction: Beneficiaries in Medicare Advantage cannot simultaneously hold Medigap policies. Individuals who switch back to Original Medicare from Medicare Advantage may face medical underwriting barriers for Medigap coverage in most states, depending on state insurance law. Only a subset of states — including Connecticut, Massachusetts, Maine, and New York — require guaranteed issue for Medigap outside the initial enrollment window.

Part D formulary volatility: Plan formularies can change annually during the annual enrollment period (October 15 – December 7), meaning a drug covered in one plan year may be placed on a higher tier or removed in the next. This creates medication continuity risks relevant to chronic disease management for seniors.

IRMAA bracket cliff effects: A $1 increase in reported income can shift a beneficiary into a higher IRMAA bracket, increasing combined Part B and Part D premiums by hundreds of dollars annually. SSA permits beneficiaries to appeal IRMAA determinations based on a life-changing event (marriage, divorce, loss of income) under the Income-Related Premium Appeals process.

Common misconceptions

Misconception 1: Medicare covers long-term nursing home care.
Medicare Part A covers skilled nursing facility care only for up to 100 days per benefit period following a qualifying 3-day inpatient hospital stay. Long-term custodial nursing home care is not a Medicare benefit; it is covered, subject to eligibility rules, under Medicaid. The distinction between these programs is addressed in detail at Medicaid and dual eligibility for seniors.

Misconception 2: There is no cost for Medicare Part A.
While most beneficiaries pay no Part A premium, those with fewer than 30 quarters of Medicare-covered employment pay a full premium of $505 per month in 2024 (CMS, 2024 Medicare Parts A & B Premiums). Part A also carries significant deductibles and coinsurance charges.

Misconception 3: Medicare Advantage is always cheaper than Original Medicare.
Medicare Advantage plans may advertise $0 premiums, but total out-of-pocket costs depend on plan-specific deductibles, copayments, coinsurance, and out-of-pocket maximums, which vary substantially across the more than 4,000 plan options available nationally (CMS Plan Finder data, 2024).

Misconception 4: Part D coverage is automatic.
Part D enrollment is not automatic for most beneficiaries. Failure to enroll during the Initial Enrollment Period (IEP) — the 7-month window around the 65th birthday — results in a permanent late enrollment penalty of 1% of the national base beneficiary premium per month of uncovered delay, calculated and added to the monthly premium for the duration of enrollment.

Misconception 5: Preventive services are fully covered under Part B.
Part B covers specified preventive services at 0% coinsurance when furnished by a participating provider and billed correctly under a preventive code. However, if a provider also addresses a separate problem during the same visit, the visit may be reclassified as diagnostic, triggering the 20% coinsurance. CMS guidance on this "preventive-turned-diagnostic" billing issue is documented in the Medicare Claims Processing Manual, Chapter 18.

Checklist or steps (non-advisory)

The following represents the structural sequence of Medicare enrollment and coverage verification events as defined by CMS and SSA administrative rules.

Enrollment and Coverage Verification Sequence

1. Confirm eligibility basis: Verify whether eligibility derives from age (65+), disability (SSDI 24-month rule), ESRD, or ALS, as each triggers different enrollment timelines and premium rules per SSA Publication No. 05-10043.
2. Identify Initial Enrollment Period (IEP): The IEP spans 7 months — 3 months before, the month of, and 3 months after the 65th birthday. Enrollment during the first 3 months activates coverage the month of the 65th birthday; later months in the IEP result in delayed coverage start dates.
3. Assess Part A premium status: Determine quarters of Medicare-covered employment using SSA records (accessible via ssa.gov my Social Security account) to establish whether Part A is premium-free or premium-based.
4. Evaluate Part B enrollment: Confirm enrollment or document qualifying coverage through an employer group health plan, which creates a Special Enrollment Period (SEP) and waives the late enrollment penalty during active coverage.
5. Select coverage path: Choose between Original Medicare (Parts A + B) with optional Medigap and standalone Part D plan, or Medicare Advantage (Part C) which bundles A + B and typically D.
6. Verify Medigap eligibility window: If Original Medicare is chosen, the 6-month Medigap Open Enrollment Period begins when Part B is effective and age 65 is reached. Outside this window, underwriting rules apply in most states.
7. Enroll in Part D or document creditable coverage: If Original Medicare is chosen without Medicare Advantage, enroll in a standalone Prescription Drug Plan (PDP) or document employer/union coverage that qualifies as "creditable" under 42 C.F.R. § 423.56 to avoid late enrollment penalties.
8. Review Low-Income Subsidy (LIS) eligibility: Determine whether income and assets fall within thresholds for the Part D Extra Help program administered by SSA, which reduces premiums, deductibles, and copayments.
9. Apply for Medicare Savings Programs (MSPs): State Medicaid agencies administer four MSP categories — Qualified Medicare Beneficiary (QMB), Specified Low-Income Medicare Beneficiary (SLMB), Qualifying Individual (QI), and Qualified Disabled Working Individual (QDWI) — that may pay Part A and B premiums, deductibles, and coinsurance.
10. Confirm annual enrollment periods: Note the Annual Enrollment Period (AEP, October 15 – December 7) for Part D and Medicare Advantage changes, and the Medicare Advantage Open Enrollment Period (January 1 – March 31) for switching from MA to Original Medicare.

Reference table or matrix

Medicare Parts A, B, C, and D: Comparative Overview