What Is Medigap? Medicare Supplement Insurance Explained

Original Medicare is often described as having "gaps" — cost-sharing obligations like deductibles, copays, and coinsurance that beneficiaries must pay out of pocket. Medigap, also called Medicare Supplement Insurance, is a category of private insurance policies specifically designed to fill those gaps. But Medigap has its own complexity, and understanding it is worth your time.

What Is Medigap?

Medigap is a type of health insurance sold by private insurance companies to supplement original Medicare (Parts A and B). It is not a standalone plan — it works alongside original Medicare, picking up some or all of the out-of-pocket costs that Medicare leaves behind.

Medigap policies are standardized by federal law. There are 10 standardized plan types, labeled Plan A through Plan N (Plans C and F are no longer available to people who became Medicare-eligible after January 1, 2020, but those who enrolled earlier can keep them). Each plan letter offers the same basic benefits regardless of which insurance company sells it — the only difference is the premium.

This standardization is enormously helpful: Plan G from Humana covers the same things as Plan G from AARP/UnitedHealthcare. You're only shopping for price and company reputation.

What Medigap Plans Cover

Different plan letters cover different combinations of Medicare cost-sharing. Here's what the core benefits address:

• Part A coinsurance and hospital costs: Most plans cover this; Part A hospital stays have significant coinsurance after 60 days.
• Part B coinsurance or copayment: The 20% you owe after Medicare pays 80% — one of the largest potential gaps.
• Blood (first 3 pints): Medicare doesn't cover the first 3 pints of blood used in a procedure.
• Part A hospice care coinsurance: Small copays for hospice services.
• Skilled nursing facility coinsurance: After day 20, you pay per-day coinsurance for SNF stays; some plans cover this.
• Part A deductible: The 2025 Part A deductible is $1,676 per benefit period — major plans like Plan G cover this.
• Part B deductible: Only Plans C and F cover this (pre-2020 eligibles only).
• Part B excess charges: If your doctor charges more than Medicare's approved amount, the excess is your responsibility unless your plan covers it (Plans F and G do).
• Foreign travel emergency: Plans C, D, F, G, M, and N cover 80% of emergency care outside the US, up to a lifetime limit.

Most popular plans: Plan G (for those newly eligible after 2020) and Plan N are the most widely chosen because of their comprehensive coverage.

What Medigap Does Not Cover

Medigap does not cover:

• Prescription drugs (you need a separate Part D plan)
• Dental, vision, or hearing (original Medicare doesn't cover these either)
• Long-term care
• Private-duty nursing
• Any Medicare Advantage costs (Medigap cannot be used alongside Medicare Advantage)

When Medigap Denies Claims

Because Medigap follows Medicare's coverage rules, most denials are either Medicare denials (where Medigap cannot pay because Medicare didn't) or administrative errors.

1. Medicare denied the underlying claim. If Medicare doesn't cover a service, Medigap won't either. Medigap supplements Medicare; it doesn't replace it. Appeal the Medicare denial first — if Medicare approves the claim, Medigap must pay its portion.

2. Coordination of benefits failure. If you have other insurance alongside Medigap, billing order must be correct: Medicare primary, Medigap secondary. If a provider submitted the bill to Medigap first, it may be rejected.

3. Non-covered services. Some people assume Medigap is a blank check. It isn't. If a service isn't covered by Medicare, Medigap won't pay for it either.

4. Billing errors. Administrative errors — wrong Medicare Beneficiary Identifier (MBI), wrong procedure codes — can result in technical denials that are correctable.

Guaranteed Issue Rights: Your Window to Enroll Without Medical Underwriting

This is one of the most important and least understood areas of Medigap law. During your Medigap Open Enrollment Period — the 6-month window that begins when you're both 65+ and enrolled in Medicare Part B — you have guaranteed issue rights. This means no insurance company can:

• Refuse to sell you any Medigap plan it offers in your state
• Charge you more because of health conditions
• Make you wait for coverage to begin

Outside this window, most states allow Medigap insurers to use medical underwriting. They can reject your application or charge higher premiums based on your health history.

Guaranteed issue also applies in specific situations:

• You lose employer coverage through no fault of your own
• Your Medicare Advantage plan leaves your area or stops participating in Medicare
• You leave a Medicare Advantage plan for the first time within 12 months of joining it (trial right)
• Your Medigap insurer goes bankrupt

If your guaranteed issue rights apply and an insurer refuses to sell you a policy or charges medically underwritten rates, file a complaint with your State Insurance Department.

How to Appeal a Medigap Denial

If you believe Medigap should have paid a bill and didn't:

Step 1: Determine whether Medicare paid. Check your Medicare Summary Notice (MSN) and your Medigap EOB. If Medicare paid its share and Medigap didn't pay the remainder it should have covered, contact the Medigap insurer.

Step 2: File a written appeal with the Medigap insurer within the timeframe listed in your denial notice (usually 60-180 days).

Step 3: If the underlying Medicare claim was denied and you believe it shouldn't have been, appeal the Medicare denial first through the Medicare appeals process. A favorable Medicare decision will carry through to Medigap.

Step 4: Contact your State Insurance Commissioner if you believe the insurer is wrongfully withholding payment.

Fight Back With ClaimBack

ClaimBack's free AI tool drafts a professional appeal letter in minutes, tailored to your insurer and denial reason. Don't let a denial be the final word.

Fight your denial at ClaimBack →

Related Reading:

• How to Write an Insurance Appeal Letter
• What Is Medical Necessity?
• Internal vs External Insurance Appeal