Medicare Advantage is your primary payer in most situations. When you enroll in a Medicare Advantage plan, it replaces Original Medicare entirely for covering your medical costs. Your doctors bill the Medicare Advantage plan directly, not Original Medicare. However, there are specific circumstances where Medicare Advantage shifts to secondary payer status, and those exceptions depend on your employment situation, employer size, and whether another type of insurance is involved.
How Medicare Advantage Works as Primary
A Medicare Advantage plan (also called Part C) is not a supplement to Medicare. It is Medicare. Private insurance companies administer these plans under contract with the federal government, and when you’re enrolled, the plan handles all your Part A and Part B benefits. If you visit a doctor, go to the hospital, or receive outpatient care, the Medicare Advantage plan processes and pays the claim first. Original Medicare does not pay anything on your behalf while you’re enrolled in a Medicare Advantage plan.
This is a key distinction from Medigap (Medicare Supplement) policies. A Medigap policy always acts as secondary insurance, picking up costs that Original Medicare leaves behind, like copays and deductibles. Medicare Advantage works the opposite way: it replaces Original Medicare rather than supplementing it. You cannot have both a Medigap policy and a Medicare Advantage plan at the same time.
When Employer Insurance Pays First
The most common situation where Medicare Advantage becomes secondary involves employer-sponsored group health insurance. The rules depend on your age, reason for Medicare eligibility, and how many people your employer has on staff.
If you’re 65 or older and still working (or covered through a working spouse), and the employer has 20 or more employees, the employer’s group health plan pays first. Medicare Advantage becomes secondary and covers remaining eligible costs after the group plan has paid its share. If the employer has fewer than 20 employees, the roles flip: Medicare Advantage pays first, and the employer plan becomes secondary.
For people under 65 who qualify for Medicare through a disability, the employee threshold is higher. An employer group plan pays first only when the employer has 100 or more employees. Below that number, Medicare Advantage is primary.
- Age 65+, employer has 20+ employees: Employer plan pays first, Medicare Advantage is secondary
- Age 65+, employer has fewer than 20 employees: Medicare Advantage pays first
- Under 65 with disability, employer has 100+ employees: Employer plan pays first, Medicare Advantage is secondary
- Under 65 with disability, employer has fewer than 100 employees: Medicare Advantage pays first
Small Employer Exception for Multi-Employer Plans
The rules get slightly more complicated when a small employer participates in a multi-employer or multiple-employer group health plan. If even one employer in the group has 20 or more employees, the standard rules apply to everyone in the plan, including workers at the smaller companies. A multi-employer plan can request an exception from CMS’s Benefits Coordination & Recovery Center to exempt workers at employers with fewer than 20 employees, but this requires a formal approval process.
Workers’ Compensation and Liability Insurance
Medicare, including Medicare Advantage, is always secondary to workers’ compensation, liability insurance, and no-fault insurance. If you’re injured on the job or in a car accident, the responsible insurer is expected to pay first. This applies even to self-insured businesses that carry their own risk rather than purchasing a traditional insurance policy.
Medicare Advantage may make what’s called a “conditional payment” while a workers’ comp or liability claim is being resolved, essentially covering your medical bills so you’re not left waiting. But once the claim settles, you’re required to cooperate with repayment efforts. The plan will contact you to investigate whether claims are related to the settlement and request reimbursement from those funds.
End-Stage Renal Disease Coordination Period
If you qualify for Medicare solely because of end-stage renal disease (ESRD) and you also have employer group health coverage, a special 30-month coordination period applies. During those first 30 months of Medicare entitlement, your employer plan pays first and Medicare is secondary. After the 30-month window closes, Medicare becomes primary.
If the basis for your Medicare eligibility changes during that period, say you turn 65 or qualify through a disability, the 30-month coordination period ends early. From that point, the standard employer-size rules described above take over. If you have more than one period of ESRD-based entitlement, each one triggers its own separate 30-month window.
What Happens Outside the Network
One important wrinkle with Medicare Advantage: if your plan is an HMO or certain types of employer-sponsored PPO, and you receive care outside the plan’s network without prior approval, it’s possible that neither the Medicare Advantage plan nor Original Medicare will cover the bill. Before seeking care from an out-of-network provider, contact your plan to confirm whether the service will be covered. This is different from Original Medicare, which generally lets you see any provider that accepts Medicare nationwide.