A broker who advises every client turning 65 to delay Medicare Part B enrollment because they have employer coverage is going to get someone a permanent penalty. The safe harbor for delaying Part B is narrower than most people assume: it requires current active employment at an employer with 20 or more employees. Everything else, including COBRA, retiree coverage, and small employer plans, sits outside that exception. Getting the employer size wrong is the source of most penalties that surface two or three years after enrollment when Medicare starts asking questions about prior creditable coverage.
Key Takeaways
- The Medicare Secondary Payer statute requires employers with 20 or more employees to offer Medicare-eligible employees the same group health plan as other employees. For those clients, employer coverage pays primary; Medicare coordinates as secondary.
- Employers with fewer than 20 employees fall below the MSP threshold. Medicare becomes primary for any employee who is Medicare-eligible by age or disability, regardless of whether the employer offers coverage.
- A client who relies on small employer coverage (fewer than 20 employees) and delays Part B enrollment is accumulating a late enrollment penalty and may have claims rejected when providers bill Medicare first and find no Part A or Part B in force.
- COBRA coverage is not the same as active employment coverage for Part B delay purposes. The day active employment ends and COBRA begins, the employer plan is no longer considered primary creditable coverage for the Part B delay exception. The Part B enrollment window starts.
- The employer size count uses the prior calendar year's average number of employees for the current plan year, not the headcount on the day the client turns 65. An employer that dipped below 20 employees in the prior year may be below the threshold even if they have hired back above 20 this year.
The Medicare Secondary Payer statute and the 20-employee threshold
The Medicare Secondary Payer statute, codified at 42 U.S.C. Section 1395y(b), establishes rules for which payer is responsible first for individuals who have both Medicare and another insurance source. For working aged individuals, the statute creates a size-based rule: employers with 20 or more employees must make Medicare-eligible employees eligible for the same group health plan under the same conditions as non-Medicare-eligible employees. That employer plan must pay primary for any enrolled Medicare-eligible employee. Medicare coordinates as secondary.
Employers with fewer than 20 employees are exempt from this requirement. They may offer a group health plan, but they are not required to make it primary for Medicare-eligible members. Under MSP rules, Medicare becomes primary for a Medicare-eligible employee at a small employer, regardless of what the employer plan says. The employer plan, if it exists, coordinates as secondary only if the plan document permits secondary coordination.
The threshold counts the number of employees, not covered lives. An employer with 19 active employees who covers 45 individuals including spouses and dependents is still below the 20-employee threshold. The 19 active employees determine the employer's MSP status.
| Scenario | Employer size | Primary payer | Part B delay safe? |
|---|---|---|---|
| Active employee, age 65+ | 20 or more employees | Employer group plan | Yes. Part B delay without penalty while actively employed. |
| Active employee, age 65+ | Fewer than 20 employees | Medicare (Part A and Part B) | No. Employer coverage does not qualify. Delaying Part B triggers a penalty. |
| Retiree with former employer retiree benefit | Any | Medicare (primary) | No. Retiree benefits are not active employment coverage. |
| Employee on COBRA after separation | Any | Medicare (primary) | No. COBRA begins the 8-month Part B SEP enrollment window. |
| Spouse covered under active employee's plan | 20 or more employees | Employer group plan (if spouse is Medicare-eligible) | Yes. Coverage as a dependent of an active employee qualifies. |
Illustrative reference. Confirm employer size and plan terms with the employer and plan document before advising on Part B enrollment timing. See 42 U.S.C. 1395y(b) for statutory basis.
The prior calendar year count matters more than the current headcount
CMS uses the employer's employment count from the prior calendar year to determine the employer's MSP status for the current plan year. An employer with an average of 22 employees in 2025 is above the 20-employee threshold for the 2026 plan year, even if they reduce staff to 16 employees in January 2026. An employer with 18 employees on average in 2025 is below the threshold for 2026 even if they hire aggressively in early 2026.
This creates a timing mismatch that catches clients. A client who joins an employer in January 2026 with 22 current employees may be at a small employer for MSP purposes if that employer averaged below 20 in 2025. The plan year that governs the MSP determination for the 2026 plan year is the 2025 headcount. Brokers cannot rely on a visual count of current staff; they need the HR-documented prior calendar year average.
COBRA is not the same as active employment coverage
The most predictable source of Part B late enrollment penalties in this category comes from clients who retire, elect COBRA, and assume COBRA is the same as their prior active employment coverage for Medicare coordination purposes. It is not.
COBRA is continuation coverage under the Consolidated Omnibus Budget Reconciliation Act, available after a qualifying event such as termination of employment, reduction in hours, or retirement. Once active employment ends and COBRA begins, the individual no longer has current employment-based group health coverage for MSP purposes. Medicare becomes primary. The 8-month Special Enrollment Period for Part B enrollment without penalty begins on the later of: the month the individual turns 65, or the month employment ends.
A client who retires at 65 in March, elects COBRA through September, and first considers Part B enrollment in October has already consumed 7 of the 8 months in the SEP window. If the SEP window closes before they enroll, they face the late enrollment penalty. Brokers who advise clients approaching retirement should schedule the Part B conversation at the same time as the COBRA conversation, not later.
When the small employer plan doesn't coordinate
A client at a small employer (fewer than 20 employees) who has Medicare-primary coverage and retains employer group coverage as secondary faces a specific practical risk: some small employer plans explicitly state in their plan document that they do not coordinate benefits with Medicare.
If the plan does not coordinate, it may deny claims that Medicare leaves to the secondary payer, citing Medicare as the responsible primary payer. The client gets the Medicare payment and nothing from the employer plan on top. For a client with a cost-sharing gap between Medicare and total billed charges, that secondary denial can leave real out-of-pocket exposure. The employer plan language controls whether this happens; brokers working with small employer clients should review the Summary Plan Description for the coordination of benefits section before advising on Medicare enrollment.
A small employer plan that does coordinate with Medicare provides secondary benefits that can be substantial. Medicare covers 80 percent of approved charges under Part B after the deductible. An employer secondary plan that covers the 20 percent coinsurance effectively functions like Medigap for that employer's active and retired workforce, at group premium rates. That arrangement is worth retaining if the plan document permits secondary coordination.
How the Part B penalty compounds over time
The Part B late enrollment penalty adds 10 percent to the standard Part B premium for each full 12-month period the individual lacked Part B and lacked qualifying employer coverage as the basis for delay. The calculation uses the standard monthly Part B premium for the year the penalty is paid, not the year the delay occurred. As the standard premium rises annually, the penalty cost in dollar terms also rises.
Example: a client who delayed Part B for 3 full years without a qualifying delay reason faces a 30 percent permanent penalty. At the 2026 standard Part B premium of $185 per month, the penalty adds $55.50 per month. Over a 20-year Medicare enrollment period, the penalty cost accumulates to $13,320 in excess premiums on top of what the client would have paid without the penalty. The penalty applies even if the client is now enrolled in an MA plan that includes Part B coverage, because the penalty attaches to the underlying Part B enrollment, not the MA plan.
The Medicare Part B late enrollment penalty guide covers the penalty calculation in full, including the interaction with Medicare Advantage plan premiums. The Medicare Initial Enrollment Period guide covers the 7-month IEP window and the Part B delay SEP timing for those with qualifying employer coverage.
The broker documentation checklist for clients turning 65
Before advising a client to delay Part B, document three things in the client file:
- Written confirmation of the employer's active employee count for the prior calendar year, confirming the employer is at or above the 20-employee threshold. HR email or a copy of the plan's SPD that references the employer's size is sufficient.
- Confirmation that the client is actively employed, not on COBRA, retiree coverage, or any other post-employment coverage arrangement.
- The client's anticipated separation date if retirement is planned. Build a calendar reminder for 30 days before the 8-month SEP window closes after that date.
If any of the three conditions cannot be confirmed with documentation, the conservative recommendation is to enroll in Part B during the initial enrollment period. The cost of enrolling in Part B when it was not strictly necessary is approximately $185 per month. The cost of failing to enroll in Part B when it was required can be a lifetime penalty and a period of claims going unpaid.
FAQ: Medicare employer coverage and the 20-employee rule
Broker questions on MSP coordination, Part B delay eligibility, and documentation requirements.
How does a broker confirm whether an employer meets the 20-employee threshold?
The employer is the most reliable source. Ask the HR department or benefits administrator to provide a written confirmation of the employer's active employee count for the current and prior plan year. The Medicare Secondary Payer threshold uses the prior calendar year's average to set the current year's primary-payer status, so an employer with 18 employees on average in 2025 is below the threshold for 2026 even if they hired additional staff. The employer's group health plan documents often state the employer's MSP status. Some carriers include a field in the Summary Plan Description noting whether Medicare is primary or secondary for enrolled Medicare-eligible members. If the employer cannot provide written confirmation, have the client call 1-800-MEDICARE and request a Benefits Coordination and Recovery Center inquiry, which can verify the employer's Medicare coordination obligation on file.
What happens when a client retires and goes on COBRA at 65?
COBRA is continuation coverage following a qualifying event such as termination of employment, reduction in hours, or retirement. It is not active employment coverage. Under the Medicare Secondary Payer statute, only coverage that is primary because of current employment counts as the basis for delaying Part B without penalty. When active employment ends and COBRA begins, that coordination status ends. The client now has COBRA as their only coverage, and Medicare is technically available but not yet elected. If the client does not enroll in Part B within the 8-month Special Enrollment Period that begins on the later of the month they turn 65 or the month the employer coverage ends as active employment coverage, they face a late enrollment penalty. Brokers who advise clients near retirement must separate the retirement date from the COBRA enrollment and set the Part B enrollment clock clearly.
What is the Part B late enrollment penalty and how long does it last?
The Part B late enrollment penalty is a permanent increase in the monthly Part B premium. The penalty adds 10 percent to the standard Part B premium for each full 12-month period the client delayed Part B enrollment without creditable employer coverage as the basis for delay. A client who delays Part B by three years without qualifying employer coverage faces a 30 percent permanent premium increase, paid for as long as they have Part B. The 2026 standard Part B premium is $185 per month. A 30 percent penalty adds $55.50 per month permanently. Over 20 years of Medicare enrollment, that accumulates to over $13,000 in excess premiums paid for the same Part B coverage. The penalty applies even when the delay was unintentional and even when the client had other coverage, unless that coverage qualifies as the specific employer-sponsored active employment exception.
Does retiree health coverage from a former employer count as primary payer coverage for delaying Part B?
No. Retiree health coverage is not current employment-based coverage for Medicare Secondary Payer purposes. The MSP statute's active employment exception applies only to coverage provided under a group health plan because of the individual's current active employment or the current active employment of a spouse. A retiree who receives employer-sponsored health benefits post-retirement is not covered under the active employment exception. Medicare is primary for that retiree, regardless of whether the employer plan covers claims. A retiree who delays Part B based on former employer retiree coverage and does not enroll in Part B within the initial enrollment period faces the late enrollment penalty. This is one of the most common Medicare enrollment mistakes in the over-65 employed and recently retired population.
What happens if a small employer plan says it does not coordinate with Medicare?
Some small employer group health plans explicitly exclude coordination of benefits with Medicare, particularly if the employer has fewer than 20 employees and believes Medicare should be primary. These plans may deny claims for Medicare-eligible members, citing Medicare as primary. If Medicare is actually primary for that employer size, the plan's decision to not coordinate is consistent with the MSP rules. However, if the employer is above the 20-employee threshold and Medicare should be secondary, a plan that refuses to coordinate as secondary is potentially violating the MSP statute. The Medicare Secondary Payer Coordination of Benefits process involves CMS and the Benefits Coordination and Recovery Center. An employer plan above the MSP threshold that refuses to pay secondary can face CMS enforcement. Brokers who discover this discrepancy when reviewing a client's group benefits should escalate it to the employer's HR department and suggest the employer's benefits counsel review the plan document for MSP compliance.
