She’s 65 and on Her Husband’s Company Plan. It’s Had 12 Employees for Years, and That Makes Medicare Primary Whether or Not She Enrolls.

A 65-year-old woman is on her husband’s group health plan through his business. He’s 58, still working, still paying toward the premium, and the coverage has been fine. She figures she can wait on Medicare until he retires. The business has had 12 employees for years. That fact is the reason she’s about to be effectively uninsured for most of her medical care, and she has no idea.

Group health plans sponsored by small employers pay secondary to Medicare for workers and spouses aged 65 and older. The test is not simply today’s payroll. It asks whether the employer had 20 or more employees for 20 or more calendar weeks in the current or preceding calendar year. A company that has run at a dozen people for years clears that test easily, and the payment order flips whether or not she signs up. If she stays off Part B, the group plan does not become primary by default.

Why the 20-Employee Line Changes the Math

Medicare Secondary Payer rules split small employers from large ones at 20 workers, measured over that weeks-based lookback. At 20 or more, the group plan is primary for active workers and 65-plus spouses, and Medicare fills gaps. Below the line, Medicare is primary. Multiemployer arrangements can complicate this further. The plan document may not spell any of it out in plain English, but the claims system enforces it the moment a bill arrives.

What happens next depends on her specific plan’s coordination-of-benefits language, and this is the part worth understanding before she needs it. Many small-employer plans pay as though Medicare already covered its share, whether or not it paid anything. Not every plan handles it identically, which is why the language matters more than the general rule.

Say her plan does subtract Medicare’s assumed share. Take an outpatient surgery where the Medicare-approved amount is $10,000. That’s the number Medicare recognizes, not necessarily what the hospital bills. Part B would normally pay roughly 80% after the $283 annual deductible, which on those figures comes to about $7,774. The group plan then covers some or all of the remainder as secondary payer.

Without Part B, the plan still processes the claim as if Medicare paid. It writes its secondary check, and the share Medicare would have covered lands on her.

No Late Enrollment Penalty

Here’s what she doesn’t have to worry about. Because her coverage comes from her husband’s current active employment, she qualifies for the Part B Special Enrollment Period. She can sign up now, or within eight months of that employment or the current-employment coverage ending, whichever comes first, with no 10%-per-year permanent late enrollment penalty. The SEP protects the deadline. It does not protect the claims.

That distinction matters, because plenty of people in this situation have heard “you have creditable coverage, you’re fine” from an HR contact or a broker picturing a 500-employee plan. That advice answers the penalty question correctly and the payment-order question wrongly. She avoids the penalty either way. She cannot avoid the secondary-payer math unless she enrolls.

The Cost of Doing It Right

The 2026 standard Part B premium is $202.90 a month, roughly $2,435 a year, plus the $283 annual deductible. Premium-free Part A generally requires 40 quarters of covered work on her own record. She can’t yet qualify through her husband’s, since a spouse’s record ordinarily requires the worker to be at least 62, and he’s 58.

IRMAA surcharges start above $109,000 single or $218,000 joint in modified adjusted gross income from two years back. CMS puts the share of Part B enrollees paying any surcharge at roughly 8%. Set that against one unplanned surgery processed with no primary insurer on file. The premium is the deal.

What to Do Sooner Than Later

None of this fixes itself, and every month she waits is a month of claims processed against a payer that isn’t there. Three moves, in order.

1. Enroll in Part B through the Special Enrollment Period. File CMS-40B with CMS-L564, the employer verification of group coverage. The window is open while he’s actively employed and for eight months after that employment or the coverage ends, whichever is first. COBRA and retiree coverage do not restart the clock.
2. Get the employee count and the payer order in writing, and ask for the coordination-of-benefits language. A statement from HR that the plan has stayed under 20 employees and pays secondary to Medicare for 65-plus spouses is the first half. How the plan calculates its share when Medicare isn’t in the picture is the half that determines her exposure. If headcount crosses the line later, the order flips.
3. Calendar the Medigap window. Her six-month Medigap open enrollment begins the month her Part B coverage starts. Inside it, insurers can’t deny her or price her up for health history. After it closes, in most states they can.

The rule was already running before she knew it existed. The enrollment is the only part still in her hands.

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