Medicare Part C: Medicare Advantage Plans

Medicare Part C — also called Medicare Advantage — is the private-plan alternative to Original Medicare that enrolls more than 33 million beneficiaries as of the Medicare & Medicaid Services enrollment data. This page covers the structural mechanics of Medicare Advantage, how plans are regulated and classified, the tradeoffs beneficiaries face, and the specific differences from Original Medicare that shape real coverage outcomes. Understanding Part C requires understanding both its federal framework and the plan-level variation that occurs within that framework.

• 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 Part C is a federal program that allows private insurance companies, approved by the Centers for Medicare & Medicaid Services (CMS), to deliver Medicare benefits to eligible beneficiaries as an alternative to Original Medicare (Parts A and B). Established under the Balanced Budget Act of 1997 and significantly restructured by the Medicare Modernization Act of 2003, Part C plans must cover all services that Original Medicare covers — with the exception of hospice care, which Original Medicare continues to pay for even when a beneficiary is enrolled in a Medicare Advantage plan (CMS Medicare Advantage Overview).

The scope of Part C extends across all 50 states, the District of Columbia, Puerto Rico, and other U.S. territories. In 2023, 51% of all Medicare-eligible beneficiaries were enrolled in a Medicare Advantage plan, according to KFF Medicare Advantage 2023 Spotlight data. Enrollment is voluntary — a beneficiary must actively choose a Part C plan during an eligible enrollment window.

The program's scope is not static. CMS releases annual call letters that set policy requirements and bid parameters for the following plan year, making the set of available plans, benefits, and cost structures a moving target that changes each January 1.

Core mechanics or structure

Medicare Advantage functions through a per-member, per-month capitated payment system. CMS pays approved private insurers a fixed risk-adjusted amount for each enrolled beneficiary. Insurers then bear the financial risk of delivering covered services within that payment, with adjustments made based on beneficiary health status using the CMS Hierarchical Condition Category (HCC) risk-adjustment model.

Plans submit bids to CMS specifying the amount they expect to need to cover the standard Medicare benefit package. If the bid is below a benchmark (the maximum CMS will pay in a county), the plan and CMS split the difference — a "rebate" that plans can use to offer supplemental benefits, reduce cost-sharing, or lower premiums. If the bid exceeds the benchmark, enrollees pay the difference as an additional premium.

Most Medicare Advantage plans bundle Part D prescription drug coverage into a single plan structure called a Medicare Advantage Prescription Drug plan (MA-PD). Plans may also offer benefits not covered by Original Medicare, including dental, vision, hearing, and fitness programs — though the scope and generosity of those extras vary significantly by plan and county.

For a broader comparison of how Part C fits within the overall Medicare structure, the key dimensions and scopes of Medicare resource provides a side-by-side framework across all parts.

Causal relationships or drivers

Enrollment growth in Medicare Advantage is driven by a convergence of regulatory, financial, and demographic factors.

Benchmark and bidding incentives. The benchmark system creates financial incentives for plans to offer zero-dollar premiums and supplemental benefits in counties where payment rates are high relative to local healthcare costs. Counties with higher benchmarks — often rural counties where the formula inflates payment — attract more competitive plan offerings.

Risk adjustment and upcoding. The HCC risk-adjustment model rewards plans financially for documenting enrollee diagnoses. CMS and the HHS Office of Inspector General have documented patterns of diagnosis coding intensity that are higher among Medicare Advantage plans than in Original Medicare, contributing to higher per-enrollee payments. The HHS OIG published findings in 2020 estimating that CMS made approximately $6.7 billion in improper payments related to unsupported diagnosis codes (HHS OIG Report OEI-03-17-00474).

Prior authorization as a utilization tool. Medicare Advantage plans are permitted to use prior authorization requirements that do not exist in Original Medicare. A 2022 Senate Finance Committee report found that prior authorization denial rates varied substantially across plans, creating access barriers that are structurally absent in fee-for-service Medicare.

Star Ratings and quality bonuses. CMS assigns Medicare Advantage plans a Star Rating of 1 to 5 based on quality metrics. Plans rated 4 stars or higher receive quality bonus payments that increase their benchmark, giving high-rated plans additional revenue to improve benefits or reduce costs.

Classification boundaries

Medicare Advantage plans are classified primarily by their network and access model:

Health Maintenance Organization (HMO). Enrollees must use in-network providers except for emergencies. Primary care physician assignment is typically required, and specialist referrals are gated through the PCP.

Preferred Provider Organization (PPO). Enrollees can use out-of-network providers but pay higher cost-sharing for doing so. No primary care physician assignment is required.

Private Fee-for-Service (PFFS). The plan sets payment terms; any provider who agrees to those terms can treat the enrollee. PFFS plans are a small fraction of total enrollment.

Special Needs Plans (SNPs). These are Medicare Advantage plans restricted to specific populations: dual-eligible beneficiaries (Dual Eligible SNPs, or D-SNPs), institutionalized individuals (I-SNPs), and those with severe chronic conditions (C-SNPs). SNPs are required to provide targeted care management. As of 2023, D-SNPs enrolled approximately 4.8 million beneficiaries (CMS SNP Data).

Medical Savings Account (MSA). These plans pair a high-deductible Medicare Advantage plan with a CMS-funded deposit to a savings account. MSA plans do not cover prescription drugs.

Tradeoffs and tensions

Medicare Advantage plans offer structural benefits that Original Medicare does not — particularly the out-of-pocket maximum, which caps annual beneficiary cost-sharing (in 2024, the in-network cap was set at $8,850 for in-network services, per CMS 2024 Medicare Advantage and Part D Rate Announcement). Original Medicare has no out-of-pocket ceiling without supplemental coverage.

However, the tradeoffs are structural and not purely financial:

Network restrictions vs. provider freedom. HMO and PPO plans limit which providers enrollees can see at in-network cost-sharing levels. Original Medicare, combined with a Medicare Supplement Insurance (Medigap) policy, provides access to any provider who accepts Medicare assignment — a far broader network.

Prior authorization burden. Services available under Original Medicare without pre-approval may require plan authorization under Medicare Advantage. This creates delays and potential denials that do not arise in Original Medicare.

Supplemental benefits vs. benefit stability. Supplemental benefits such as dental and vision are offered at the plan's discretion and change annually. A benefit present in one plan year is not guaranteed in the next.

Geographic portability. Original Medicare covers services nationwide. Medicare Advantage plans generally restrict coverage to a service area, which creates complications for beneficiaries who travel or split time between states.

Plan exit risk. CMS permits plans to exit markets or reduce service areas. Beneficiaries in exiting plans are granted a Special Enrollment Period but must find a new plan mid-cycle.

The Medicare Advantage vs. Original Medicare comparison page covers these tradeoffs in greater depth.

Common misconceptions

Misconception: Medicare Advantage plans are part of the government.
Medicare Advantage plans are operated by private insurers under contract with CMS. CMS approves, oversees, and pays these plans, but the plans themselves — including their networks, formularies, and authorization policies — are managed by private entities.

Misconception: Zero-premium means no cost.
A plan with a $0 monthly premium still collects the standard Part B premium from the beneficiary (the standard Part B premium in 2024 was $174.70/month per CMS). Cost-sharing applies at the point of service.

Misconception: Medicare Advantage must cover everything Original Medicare covers.
While plans must cover all Part A and Part B services, coverage rules — including the specific drugs on a formulary and the prior authorization requirements — differ by plan and can change yearly.

Misconception: Medigap plans can be purchased alongside Medicare Advantage.
Medigap plans are prohibited by federal law from covering cost-sharing for Medicare Advantage benefits. A beneficiary enrolled in Medicare Advantage cannot use a Medigap policy for that plan's cost-sharing. If a beneficiary switches back to Original Medicare, Medigap guaranteed-issue rights do not automatically apply — underwriting may be required depending on the state.

Misconception: Medicare Advantage enrollment is permanent.
Beneficiaries can leave Medicare Advantage and return to Original Medicare. The Medicare enrollment periods framework includes the Annual Enrollment Period (October 15 – December 7) and the Medicare Advantage Open Enrollment Period (January 1 – March 31) specifically for this purpose.

Checklist or steps (non-advisory)

The following sequence describes the components a beneficiary must verify when evaluating a Medicare Advantage plan:

1. Confirm eligibility. The beneficiary must be enrolled in both Medicare Part A and Part B, and must reside in the plan's service area. Full Medicare eligibility requirements apply.
2. Verify plan availability. Use the Medicare Plan Finder (CMS tool) to identify plans available in the beneficiary's zip code.
3. Review the Summary of Benefits. Every plan must publish a standardized Summary of Benefits document listing premiums, deductibles, cost-sharing, and covered services for the plan year.
4. Review the Evidence of Coverage (EOC). The EOC is the full contractual document governing the plan. It specifies prior authorization requirements, formulary tiers, and appeals procedures.
5. Check the provider directory. Confirm that the beneficiary's current providers — primary care, specialists, hospitals — are listed as in-network participants and accept the plan. Provider directory accuracy is a known compliance issue; direct provider confirmation is the reliable method.
6. Review the formulary. Confirm all current medications are covered, at which tier, and with what restrictions (prior authorization, step therapy, quantity limits).
7. Calculate projected annual costs. Sum premiums, expected cost-sharing based on utilization patterns, and any drug costs under the plan formulary.
8. Note the out-of-pocket maximum. Identify whether the cap applies to in-network services only or to both in- and out-of-network services.
9. Confirm the enrollment deadline. Identify which enrollment period applies and the specific deadline. Late enrollment in Medicare can carry long-term consequences — Medicare late enrollment penalties are calculated as permanent premium increases.
10. Retain documentation. Keep the enrollment confirmation, EOC, and Summary of Benefits from the effective date of enrollment.

For assistance navigating these steps, the how to get help for Medicare resource lists publicly available counseling programs including State Health Insurance Assistance Programs (SHIPs).

Reference table or matrix

Medicare Advantage Plan Type Comparison

Key Annual Cost Parameters (2024)