Coordinating Medicare With Employer Coverage on SSDI

When SSDI brings Medicare together with an employer health plan, who pays first depends on the size of the employer, whose job provides the coverage, and your age. Before 65, an employer group plan pays ahead of Medicare only if the employer has 100 or more employees. At 65 or older, the cutoff drops to 20 employees.

These rules come from the official CMS booklet How Medicare Works With Other Insurance (CMS product 02179, February 2026 edition) and from Medicare.gov's coordination pages. Getting them right matters, because claims sent to the wrong payer first get delayed or denied. This article covers coordination once Medicare is in the picture, not the SSDI application process itself.

Coordination of benefits, in plain English

Each type of coverage you have is called a payer. The primary payer pays its share of a bill first, up to the limits of the policy. The secondary payer only steps in for costs the primary payer left behind, and only if the service is covered.

Tell every doctor and hospital about all of your coverage, because Medicare does not automatically know you have an employer plan. If you are unsure who pays first in your situation, call the Benefits Coordination and Recovery Center at 1-855-798-2627, the number CMS provides for exactly this question.

The 100-employee rule while you are under 65

If you have Medicare because of disability and you have group coverage through your current employer, or through the current employer of a spouse or family member, the employer's size controls:

• 100 or more employees, or membership in a multi-employer group where at least one company has 100 or more employees: the group health plan pays first, and Medicare pays second
• Fewer than 100 employees and no such multi-employer group: Medicare pays first, and the group plan pays second

The CMS booklet gives the example of a husband with disability Medicare whose wife works at a company with 120 employees. Her group plan pays first for him, and Medicare pays second. The booklet also notes that large group health plans cannot treat any plan member differently because the member is disabled and has Medicare.

A practical wrinkle: if your group plan is the secondary payer, the plan may require you to be enrolled in Part B before it will pay anything. The CMS booklet flags this directly, so ask your benefits administrator.

When Medicare pays first before 65

Medicare pays first before age 65 in two common situations. One is the small employer just described. The other is coverage from a former employer, such as retiree coverage, which the booklet says is not based on active employment.

If you have retiree coverage, read the fine print before declining any part of Medicare. The booklet warns that retiree coverage might not pay for periods when you were eligible for Medicare but not signed up, and that you may need both Part A and Part B to get full benefits from the retiree plan.

The first 24 months, before Medicare starts

Coordination questions usually arrive around month 24 of SSDI entitlement. Before that point, you are not yet eligible for Medicare, so your group plan is the only payer you have. The CMS booklet notes there is no 24-month wait for people with ALS.

Once you receive disability benefits from SSA or the Railroad Retirement Board for 24 months, you are signed up for Medicare automatically. At that moment the coordination rules above start applying, whether or not anyone explained them to you.

Deciding whether to take Part B at month 24

If your employer plan will pay first, for instance at a large employer, you may be tempted to decline Part B and its $202.90 monthly premium in 2026. Medicare.gov provides a safety valve for that choice: a special enrollment period that lets you sign up for Part B later without any late enrollment penalty.

The enrollment period runs any time you are covered by a group health plan through the current employment of you or your spouse, and it continues for up to 8 months after that employment or that group coverage ends, whichever comes first. SSA's Program Operations Manual System covers these enrollments in POMS HI 00805.275.

When you do sign up, you will use Form CMS-40B for the Part B application, and your employer may need to complete Form CMS-L564 confirming your group coverage. Ask your employer for that form while the employment relationship still exists, because tracking it down later is one of the most common snags.

One caution: the window is tied to active employment. COBRA continuation coverage is not coverage based on current employment, so it does not keep this signup window open. Calendar the 8 months carefully.

How COBRA and retiree coverage coordinate

If you have COBRA and Medicare, the CMS booklet states that Medicare pays first when you have Medicare because you are 65 or older or because of disability. The exception is ESRD: with Medicare due to end-stage renal disease, COBRA pays first during a coordination period of up to 30 months, after which Medicare pays first. That same 30-month coordination rule applies to group and retiree plans of any size during an ESRD-based entitlement.

The booklet also warns that your COBRA election period, your Part B enrollment period, and your Medigap open enrollment period can carry different, overlapping deadlines. A decision about one can cost you rights under another, so talk it through with a SHIP counselor before electing COBRA.

On the drug side, if your COBRA includes creditable prescription drug coverage, you get a special enrollment period to join a Medicare drug plan without penalty when COBRA ends.

What changes when you turn 65

At 65 the coordination test shifts from the 100-employee rule to a 20-employee rule. With 20 or more employees, the group plan pays first and Medicare second. With fewer than 20, unless a multi-employer exception applies, Medicare pays first.

The booklet adds two protections that arrive at 65. Employers with 20 or more employees must offer employees aged 65 and over the same health benefits, under the same conditions, offered to younger employees, and the same rule applies to covered spouses. And if you decline employer coverage when it is first offered, the booklet warns you might not get another chance to join later, so think before you opt out.

Drug coverage and other traps

A few coordination details from the CMS booklet and other official sources catch SSDI beneficiaries off guard:

• Your employer or union must notify you each year whether its drug coverage is creditable. Keep those letters, because you can delay joining Part D without penalty only while you have creditable coverage and no gap of 63 or more days.
• Before adding Medicare drug coverage on top of employer or union coverage, check with the benefits administrator. The booklet warns that adding Part D can cost you employer benefits, including non-drug coverage, for you and your dependents.
• With an employer HMO or PPO that pays first, getting care outside its network can mean neither the plan nor Medicare pays.
• If you contribute to a health savings account, you must stop contributing once any part of Medicare starts, including the Part A you receive automatically after 24 months of SSDI. The IRS explains this rule in Publication 969.
• If your former employer offers retiree coverage, you may be denied it later if you were not in the plan while still working, another reason to check before dropping anything.

Frequently asked questions

Who pays first if my employer has 50 employees? With disability Medicare before 65, Medicare pays first unless the employer has 100 or more employees. After you turn 65, the cutoff drops to 20 employees, so at 50 employees your group plan becomes primary.

Can I delay Part B while I have employer coverage? Yes, if the coverage comes from the current employment of you or your spouse. You can sign up later without penalty during a special enrollment period that runs while you are covered and for up to 8 months after the employment or coverage ends.

Does COBRA count as employer coverage for delaying Part B? No. COBRA is continuation coverage after employment ends, not coverage based on active work. Medicare generally pays first when you have COBRA, and the booklet How Medicare Works With Other Insurance warns that COBRA, Part B, and Medigap deadlines can interact in ways that cost you rights.

This article is general information, not legal advice. The Part B premium cited is the standard 2026 amount from CMS, and coordination rules can carry exceptions by plan type. Consult a qualified representative or your State Health Insurance Assistance Program (SHIP) about your specific situation.