Medicare’s four parts combine to form your health coverage, but the way they interact is what most people miss. Part A covers hospital stays, Part B covers outpatient and physician services, Part D adds prescription drug coverage, and Part C (Medicare Advantage) bundles all three into one private plan. Medigap sits alongside Original Medicare to fill the cost gaps Parts A and B leave behind. Here is how these pieces fit together:
- Original Medicare = Part A + Part B, the foundation every beneficiary starts with
- Add Part D to Original Medicare for standalone prescription drug coverage
- Add Medigap to Original Medicare to cover coinsurance, deductibles, and copayments
- Medicare Advantage (Part C) replaces Original Medicare with a bundled plan that includes Parts A and B and usually Part D
- You must have both Parts A and B before joining any Medicare Advantage plan
- Medigap cannot be paired with Medicare Advantage; the two are mutually exclusive
- When you carry other insurance alongside Medicare, coordination of benefits rules determine which plan pays first
The core choice every beneficiary faces is this: stay with Original Medicare and layer on Medigap and Part D, or switch to Medicare Advantage and get everything bundled. Neither path is universally better. It comes down to your priorities.
How each Medicare part covers you and what it costs
Understanding each part individually makes the coordination picture much clearer.
Part A: hospital coverage
Part A covers inpatient hospital care, skilled nursing facility stays, hospice, and some home health services. Most people pay no premium for Part A because they or their spouse paid Medicare taxes for at least 10 years while working. You do, however, face a per-benefit-period deductible for hospital stays, plus daily coinsurance for extended inpatient care.
Part B: outpatient and physician services
Part B picks up where Part A leaves off. It covers doctor visits, outpatient procedures, preventive screenings, durable medical equipment, and home health care not tied to a hospital stay. After you meet your annual Part B deductible, you typically pay 20% of the Medicare-approved amount for covered services, with no annual cap on that 20% unless you have supplemental coverage.
Part D: prescription drug coverage
Part D plans cover prescription drugs through private insurers that follow CMS rules. Each plan uses a formulary, a tiered list of covered drugs where lower tiers cost less out of pocket. If you stay with Original Medicare, you enroll in a standalone Part D plan separately. Most Medicare Advantage plans already include Part D, so you would not need a separate drug plan.
Pro Tip: Compare formularies before picking a Part D plan. A plan with a lower premium can cost you more overall if your specific medications sit in a higher tier.
Medigap: filling the gaps in Original Medicare
Medigap policies are sold by private insurers and are standardized by letter (Plan G, Plan K, and so on). They cover costs Original Medicare leaves on the table, primarily that 20% Part B coinsurance, Part A deductibles, and hospital copayments. Medigap does not cover prescription drugs, long-term care, dental, or vision. Critically, you can only buy Medigap if you have Original Medicare. It cannot be used with Medicare Advantage.
Medicare Advantage (Part C): the bundled alternative
Medicare Advantage plans must cover everything Original Medicare covers, and most add benefits like dental, vision, hearing, and fitness programs. The trade-off is network restrictions. Unlike Original Medicare, which lets you see any Medicare-accepting provider nationwide, Medicare Advantage typically limits you to a plan network. Many plans also require prior authorization before covering certain services or procedures.
On costs, Medicare Advantage sets an annual out-of-pocket maximum on covered services. Once you hit that limit, the plan covers 100% for the rest of the year. Original Medicare has no such cap without supplemental coverage, which is why Medigap matters so much for people who stay with Original Medicare.
Enrollment rules, switching windows, and other insurance
Getting the timing right protects you from coverage gaps and unexpected bills.
The enrollment sequence matters
You must enroll in both Part A and Part B before you can join a Medicare Advantage plan. There is no shortcut. Once you have Parts A and B, you choose your path: Original Medicare with optional Medigap and Part D, or Medicare Advantage.
- Initial Enrollment Period: a seven-month window around your 65th birthday to sign up for Parts A and B
- Annual Enrollment Period (October 15 through December 7): you can switch between Original Medicare and Medicare Advantage, or change Part D plans, each year
- Medicare Advantage Open Enrollment (January 1 through March 31): if you are already in a Medicare Advantage plan, you can switch to a different Advantage plan or return to Original Medicare
Switching between plans is allowed annually, but returning to Original Medicare after Medicare Advantage can create a Medigap problem. Outside your initial open enrollment window, Medigap insurers in most states can use medical underwriting, meaning they can charge more or deny coverage based on your health history.
Late Part B enrollment creates real risk
Failing to enroll in Part B when you are first eligible can cause secondary insurers, including retiree plans and employer coverage, to deny claims. They may refuse to pay because Medicare should have been primary. That leaves you holding the bill.
When you have other insurance alongside Medicare
Coordination of benefits rules set the payment order when Medicare is not your only coverage. The primary payer pays first up to its coverage limits, then the secondary payer covers remaining costs. Who pays first depends on your situation:
- Retiree coverage: Medicare pays first
- Active employer coverage (employer with 20 or more employees): your group health plan pays first
- Active employer coverage (employer with fewer than 20 employees): Medicare pays first
- Medicaid: Medicare always pays first
If your primary insurer does not process a claim within 120 days, Medicare may make a conditional payment to prevent delays in your care, then recover those funds from the primary payer later.
What Paul Barrett and Paulbinsurance want you to know
Paul Barrett has worked as a Medicare principal agent since 2007, and the most common mistake he sees is beneficiaries choosing a plan without fully understanding the cost trade-offs between Original Medicare and Medicare Advantage.
The fundamental question is whether you prioritize provider freedom or cost predictability. Original Medicare gives you access to any Medicare-accepting provider in the country, which matters if you travel frequently, see specialists at major academic medical centers, or simply want no network restrictions. Medicare Advantage can cap your annual out-of-pocket costs and bundle in extras like dental and vision, but those benefits come with network rules and prior authorization requirements that can slow or complicate care.
Pro Tip: Before choosing Medicare Advantage, call your current doctors and ask if they accept the specific plan you are considering. “Accepts Medicare” and “in-network for Plan X” are not the same thing.
One point that trips up many beneficiaries: Medigap and Medicare Advantage are not interchangeable add-ons. They represent two completely different strategies. You pick one path and commit. Trying to hold both is not just inadvisable; it is prohibited. CMS defines “medically necessary” as the standard for what Medicare covers, but Medicare Advantage plans can add prior authorization layers that Original Medicare does not. Verifying coverage before a procedure, not after, prevents surprise bills.
Paulbinsurance’s approach starts with education. Knowing how Medicare benefits coordinate, which plan pays first, and what each part actually covers puts you in control of the decision rather than guessing at it.
Paulbinsurance makes Medicare coverage decisions less stressful
Independent guidance on Medicare is hard to find. Most people get a stack of mailers and a lot of conflicting advice.
Paulbinsurance has been helping seniors cut through that noise since 2007. As independent agents, the team is not tied to any single carrier, so the focus stays on finding the right fit for your situation, whether that is a Medicare Advantage plan, a Medigap policy, a standalone Part D plan, or a combination. Paul Barrett and the team walk you through how each part of Medicare applies to your doctors, your prescriptions, and your budget before you commit to anything. If you are approaching 65 or already enrolled and unsure whether your current coverage still fits, get personalized Medicare guidance from a team that has been doing this for nearly two decades.
Key Takeaways
Medicare’s four parts coordinate to give you a complete coverage picture, and the path you choose between Original Medicare and Medicare Advantage shapes every cost and access decision that follows.
| Point | Details |
|---|---|
| Original Medicare foundation | Parts A and B cover hospital and outpatient care; you add Part D and Medigap separately. |
| Medicare Advantage bundles coverage | Part C includes Parts A, B, and usually Part D in one plan, often with dental, vision, and fitness extras. |
| Medigap exclusivity | Medigap can only supplement Original Medicare; it cannot be paired with Medicare Advantage. |
| Enrollment timing protects you | Late Part B enrollment can cause secondary insurers to deny claims, leaving you with uncovered costs. |
| Paulbinsurance | Independent agents help you compare Medicare Advantage, Medigap, and Part D options based on your specific doctors, drugs, and budget. |
