Medicare and Employer Health Insurance

How to coordinate Medicare with your group health plan — and avoid costly mistakes.

INTRODUCTION

If you're approaching Medicare eligibility while still covered by an employer group health plan — yours or your spouse's — one of the bigger questions you'll face is whether to enroll in Medicare, delay it, or take only certain parts. For most people the answer comes down to one factor: the size of the employer providing your group coverage. The same is true if you're transitioning to COBRA after leaving a job.

This page focuses primarily on those two situations, since they cover the vast majority of cases. We also briefly cover the rules that apply when Medicare eligibility is based on disability or End-Stage Renal Disease (ESRD), since those rules are different.

The Most Important Factor: Employer Size

For most people becoming Medicare-eligible while still working (or while a spouse is still working), the single biggest factor in deciding what to do is the size of the employer. The rules switch at the 20-employee threshold for age-based Medicare eligibility:

• Large employer (20+ employees): Group plan stays primary, you can typically delay Medicare

• Small employer (under 20 employees): Medicare becomes primary, you generally need to enroll

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The rules look slightly different in the less common situations — disability and ESRD — and we cover those briefly toward the end of this page.

Coordination of Benefits — Quick Reference

The chart below shows who pays primary in each common situation. The rest of the page focuses on the two most common: age-65 enrollment alongside an active employer plan, and how COBRA changes the picture.

If You're Medicare-Eligible by Age (Turning 65)

This is the most common scenario by far. The right move depends entirely on the size of the employer providing your group health coverage — the rules change at the 20-employee mark.

Large Employer — 20 or More Employees

Primary: Group health plan. Secondary: Medicare.

In this situation, you generally don't need to enroll in Part A or Part B at 65 because the group plan continues to be your main coverage. Some people sign up for Part A anyway because it's typically premium-free — but be aware that any Medicare enrollment, even Part A only, ends your ability to contribute to a Health Savings Account (HSA). If HSA contributions are part of your strategy, see our Medicare and HSAs page before enrolling in any portion of Medicare.

Don't enroll in Part B in this situation unless you have End-Stage Renal Disease. Part B carries a monthly premium and you can defer it without penalty when covered by a large group plan based on active employment. When you're eventually ready to add Part B (typically when you retire or lose group coverage), you'll use a Special Enrollment Period — see our SEP application guide.

Small Employer — Under 20 Employees

Primary: Medicare. Secondary: Group health plan.

The order flips at small employers. Medicare becomes your primary coverage, with the group plan paying secondary. In this case, you need both Part A and Part B to keep things working properly. If you skip Part B, your group plan may pay claims as if Medicare were already covering its share — leaving you on the hook for the unpaid portion of outpatient bills.

Our recommendation when you're covered by a small employer plan is to enroll in both Part A and Part B at 65. Some small-employer plans even offer a premium discount once Medicare is primary, so it's worth asking your benefits coordinator about that.

One possible exception worth knowing about: some small employers join an association group health plan that pools coverage across many employers. In some cases, this can qualify the plan as a large employer for coordination purposes — which would flip the order back. Check with your plan administrator if you're unsure how your specific plan is classified.

Medicare and COBRA Continuation Coverage

COBRA is an extension of your prior employer group coverage that you can elect after leaving employment. It typically lasts 18 months (occasionally up to 36 months in certain situations). The critical thing to understand is that COBRA is not considered active employer coverage for Medicare purposes — so the rules that let you delay Part B while still working don't apply once you're on COBRA.

COBRA — Coordination With Medicare

Primary: Medicare. Secondary: COBRA.

If you're Medicare-eligible and on COBRA, you need both Part A and Part B. COBRA does not allow you to delay Part B enrollment without penalty the way active employment does — so don't assume you have the same Special Enrollment Period flexibility just because you have COBRA coverage. Missing Part B during a COBRA period can result in late enrollment penalties and gaps in coverage.

Less Common Situations: Disability and ESRD

The coordination rules differ for people who become Medicare-eligible due to disability or End-Stage Renal Disease (ESRD). Both situations are less common than age-based eligibility, but the rules below apply when they come up.

Medicare Eligibility Based on Disability

The size threshold here is different — instead of 20 employees, the cutoff is 100:

• 100 or more employees: Group plan is primary, Medicare is secondary.

• Fewer than 100 employees: Medicare is primary, group plan is secondary.

  
  

Medicare Eligibility Based on ESRD (Kidney Failure)

ESRD has its own rules, which apply regardless of employer size. Medicare typically begins in the fourth month of dialysis treatment. From there, a 30-month coordination period applies:

• First 30 months: Group plan (or COBRA) is primary, Medicare is secondary.

• After 30 months: Medicare becomes primary, with the group plan or COBRA as secondary.

The Full Decision Checklist

The coordination rules above tell you what's required, but the question of whether to enroll (and in what) often comes down to broader cost and lifestyle factors. Here's the full list of things to weigh:

• The number of employees at your (or your spouse's) employer

• Your basis for Medicare eligibility — age, disability, or ESRD

• The type of employer health plan you have (PPO, HMO, High Deductible Health Plan, etc.)

• Whether you're contributing to a Health Savings Account — and how much that tax benefit is worth to you

• How the group plan's premium compares to what you'd pay for Medicare plus supplemental coverage

• Your income — which determines whether you'll owe IRMAA surcharges on Part B and Part D

• How the group plan's deductible and out-of-pocket maximum compare to Medicare with a supplement

• Whether you're a low or high user of medical services in a typical year

• Whether you're also covering a younger spouse or dependent children on the group plan

• When you plan to retire or otherwise lose group coverage

• Whether the group plan's prescription drug benefits are creditable compared to Medicare Part D — non-creditable coverage can trigger a Part D late enrollment penalty later

• Any extras the employer provides — HSA contributions, premium reimbursements, retiree health benefits, and so on

Helpful Reference Links

• Working Past 65 — Medicare and Employer Coverage at medicare.gov/basics/get-started-with-medicare/medicare-basics/working-past-65

• Medicare and ESRD (End-Stage Renal Disease) at medicare.gov/basics/end-stage-renal-disease

• Medicare and COBRA — 7 Important Facts at medicare.gov/supplements-other-insurance/how-medicare-works-with-other-insurance/cobra-7-important-facts

• Special Enrollment Period (SEP) Application Guide — our companion page on enrolling in Part B after delaying

• Medicare and Health Savings Accounts (HSAs) — our companion page on HSA contribution rules

Have Questions?

If you have any questions about the information on this page, feel free to reach out — we're happy to help point you in the right direction.

(512) 298-5404 / gofouroaks.com

This page is provided for informational purposes only and does not constitute legal, tax, or financial advice. Four Oaks is not affiliated with the Social Security Administration or the federal Medicare program.