Enrollment

Small Employer: Medicare Should Have Paid First

Name: Small Employer: Medicare Should Have Paid First
Creator: Barley Billing Team
Published: 2026-09-14

Written by Barley Billing Team, Medicare Billing Experts | Fact-checked against primary CMS sources | Last reviewed September 14, 2026

What This Means

When you have both Medicare and a health plan through a job, one of them pays the bill first and the other picks up what is left. Which one goes first is not a choice. It is set by rule, and the rule turns on how many people the employer has.

If the employer is large, the work plan pays first and Medicare pays second. If the employer is small, it is the other way round: Medicare pays first, and the work plan pays second.

That reversal is where the money goes missing, and it is easiest to see with a number. Say a test costs $1,000. In a small-employer situation Medicare is meant to pay its share first — but if you never signed up for Part B, Medicare paid nothing at all, because you were not in it. Your work plan then paid only the smaller share it owes as the second payer. The difference between those two amounts did not disappear. It came to you, as a bill.

People often find this out years in, after a lot of those bills. What matters then is what can still be recovered, and the honest answer has two halves: fixing it from here is straightforward. Recovering the past turns on one thing you may not be able to get.

Why This Happens

Nobody sets out to get this wrong. It happens because everyone involved has a reasonable-sounding position:

You had coverage, it was good coverage, and paying a second premium for Part B looked like waste.
Your employer’s HR, if the business is small, may never have administered this before.
Your doctors’ billing offices submitted to the insurance you gave them. They have no way of knowing Medicare should have been billed first.
The plan paid, so nothing looked broken — until the amounts got large enough to notice.

Check One Thing Before Anything Else: Is It Really a Small-Employer Plan?

The Medicare Secondary Payer rules for people on Medicare because of age apply to group health plans of an employer of 20 or more employees, and an employer counts as having 20 or more “if the employer has 20 or more employees for each working day in each of 20 or more calendar weeks in the current calendar year or the preceding calendar year.” Below that, Medicare is primary. The citation is 42 CFR 411.170, and it is worth reading — other people have written the rule out carefully and we are not going to restate it at length.

Two warnings, and the second one decides whether this page applies to you at all.

It is not a headcount. Part-time employees count, and the test looks across two calendar years. The threshold is also different if your Medicare is based on disability rather than age, where the figure is 100 and the formula differs again. Do not work it out yourself and do not ask a colleague. Ask the benefits administrator, in writing, whether the plan pays primary or secondary to Medicare.

If the plan is run jointly with other employers, the count is not yours alone — and the default goes against you. The regulation puts you in the Medicare-secondary group if you are covered by a plan of an employer with at least 20 employees, “including a multi-employer plan in which at least one of the participating employers meets that condition”. So a five-person business buying into a large joint plan is normally a Medicare-secondary situation, not a Medicare-primary one. There is an exception that flips it back — but the plan has to request it from CMS and identify the people it covers, and many never do. If the exception was never requested, the joint plan pays first, Medicare is secondary, and there is nothing here to reopen. Establish this before you go any further.

What Can Actually Be Recovered, and the Six-Month Clock

If you were genuinely in the small-employer situation, the fix has two parts and the second depends entirely on the first.

Part one: getting Part B. Straightforward, and it fixes everything from now on.

Part two: getting that Part B backdated — that is, getting Medicare to say your cover actually started on a date in the past. This is everything as far as old bills are concerned, and it is harder than most of what you will read online suggests.

The Special Enrollment Period for employer misinformation — form CMS-10797, the one everyone reaches for — does not backdate anything. The regulation says entitlement under it “begins the first day of the month following the month of enrollment.” It also only covers misinformation received on or after January 1, 2023, which rules out most people who discover this years later. It is still well worth using: it waives the wait and it waives the late enrollment penalty. It just will not reopen a single old claim.

The route that can backdate is equitable relief, and it is available only where the error came from the government — Social Security, Medicare, or someone acting for them — not from an employer. Where it applies, Social Security may designate an enrollment period and “an entitlement period based on that enrollment period”. Ask for it by name, and ask early, because everything below depends on it.

If Medicare does backdate your cover, the one-year limit gives way — for six months. Medicare’s ordinary limit is one calendar year from the date of service, and most billing offices will tell you, correctly, that anything older is gone. There is a written exception for exactly this situation: you were not entitled to Medicare when the service happened, and you were later told, in writing, that your Medicare actually started on or before that date.

But the exception is not open-ended, and this is the part that catches people. The extended window runs only through the last day of the sixth calendar month following the month in which you or your doctor’s office were told about the backdated start date. The letter that solves your problem also starts a six-month clock. Do not spend two of those months getting organised.

And it is a decision someone makes, not something you are simply owed: the regulation is framed throughout as “if CMS or one of its contractors determines” the conditions are met. It can be refused.

What To Do Next

Get the employer’s answer in writing. Ask your benefits administrator whether the plan pays primary or secondary to Medicare for employees over 65, and whether it is a multi-employer plan. An email is enough. If the answer is “secondary to Medicare”, you have a case; if it is “primary”, you do not, and the rest of this page is not for you.
Enrol in Part B now, whatever else happens. Call Social Security on 1-800-772-1213. If your employer or its plan misinformed you on or after January 1, 2023, ask for the Special Enrollment Period on form CMS-10797 — it waives the wait and the penalty. It will not backdate your coverage.
Ask about backdating separately, and by name. Ask Social Security whether equitable relief is available in your case, and be clear about who gave you the wrong information. If it came from the government, say so and give dates. If it came only from your employer, ask anyway, but do not plan around getting it.
If backdating is granted, move immediately. Get the effective date in writing, note the date on the notification, and count six calendar months from the end of that month. That is your window for every provider.
Then go to each billing office with the citation. Ask them to refile the claims to Medicare as primary, and give them the reason the one-year limit does not bar it: 42 CFR 424.44(b)(2), retroactive Medicare entitlement, with the six-month extension at (b)(5)(ii). Expect to explain it — it is a real provision and it is rarely used.
Tell your employer’s plan what has changed. Once Medicare pays as primary, the plan’s liability is recalculated. It may owe you more; it may also have overpaid and want the difference back.
Keep every explanation of benefits you already have. The ones showing the plan paying as a second payer are the evidence of what went wrong.
Get free help, and get it early. Your state’s SHIP gives free Medicare counselling with no connection to any insurer, at shiphelp.org. Bring them this before the six months start, not during.

Sources

42 CFR 411.170 — the Medicare Secondary Payer working-aged provisions apply to group health plans “of at least one employer of 20 or more employees”, and the definition: “20 or more employees for each working day in each of 20 or more calendar weeks in the current calendar year or the preceding calendar year.”
42 CFR 411.172 — paragraph (a)(3)(i) puts you in the Medicare-secondary group if covered by a plan of an employer with at least 20 employees, “including a multi-employer plan in which at least one of the participating employers meets that condition”; paragraph (b) is the exception the plan must itself request for its small-employer members; paragraph (c) covers an employee who refuses the employer’s plan, for whom Medicare is primary and the plan “may not offer that individual coverage complementary to Medicare.”
42 CFR 424.44 — the filing limit: “the claim must be filed no later than the close of the period ending 1 calendar year after the date of service.” Paragraph (b)(2) extends it where the beneficiary “was not entitled to Medicare” when the service was furnished and “subsequently received notification of Medicare entitlement effective retroactively to or before the date of the furnished service.” Paragraph (b)(5)(ii) is the length of that extension — “through the last day of the sixth calendar month following the month in which either the beneficiary or the provider or supplier received notification of Medicare entitlement effective retroactively to or before the date of the furnished service.” Every branch of (b) is conditioned on CMS “or one of its contractors” so determining.
42 CFR 407.23 — the exceptional-conditions Special Enrollment Periods. Paragraph (c) is the one for employer or health-plan misrepresentation, requested on form CMS-10797; its paragraph (c)(3) is why it cannot backdate — “Entitlement begins the first day of the month following the month of enrollment”. Paragraph (a) limits all of them to “exceptional conditions that took place on or after January 1, 2023”. See also 42 CFR 406.27 for the Part A equivalents.
42 CFR 407.32 — equitable relief, available where enrolment or non-enrolment is wrong “because of the error, misrepresentation, on [sic] inaction of a Federal employee or any person authorized by the Federal Government to act in its behalf”. It is the route that can produce backdating, because the relief may include “(b) Designation of an entitlement period based on that enrollment period”. It does not reach an employer’s error.
CMS: Medicare Secondary Payer — how coordination of benefits is recorded and corrected.

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Frequently Asked Questions

How do I know if my employer is under the threshold?

Do not try to count heads. The test is not a headcount on any given day — it asks whether the employer had 20 or more employees for each working day in each of 20 or more calendar weeks in this year or last year, and part-timers count. And if your employer buys into a plan run jointly with other employers, the count is not yours alone. Ask your benefits administrator or HR to tell you in writing whether the plan pays primary or secondary to Medicare. They already know, because the plan has to.

Aren't my old claims too late to refile?

It depends entirely on whether you can get Medicare backdated, and that is the hard part — not the filing limit. Medicare's ordinary limit is one calendar year from the date of service, and there is a written exception for exactly this situation: you were not entitled to Medicare when the service happened, and you were later notified that your entitlement is retroactive to or before that date. But the extension is not open-ended. It runs only through the last day of the SIXTH calendar month after the month you or your provider were notified of the backdated entitlement. Six months, starting from a letter.

Will my employer plan take back what it paid?

It may recalculate. If it has been paying as though it were primary, it may have overpaid and can seek the difference once Medicare pays its share. Usually the net result is still strongly in your favour, but ask before assuming the whole Medicare payment is yours to keep.

Nobody told me I needed Part B. Does that help?

It helps with the future, not the past. The Special Enrollment Period on form CMS-10797 exists for people misinformed by an employer or its health plan, and it waives both the usual wait and the late enrollment penalty — but coverage starts the month after you enrol, not in the past, and it only covers misinformation received on or after January 1, 2023. Getting Medicare BACKDATED is a different and harder ask: it normally needs equitable relief, which Social Security can grant only where the wrong information came from the government rather than from an employer.

What if I turned the employer plan down entirely?

Then Medicare is your primary payer, and the rules bar the employer's plan from selling you anything designed to wrap around Medicare. That is worth knowing before anyone offers you a supplement through work.

This information is for educational purposes only and is not legal or medical advice. Always verify with your doctor's office and insurance company.