Medicare Coordination of Benefits: Primary vs Secondary Payer Rules
How Medicare Decides Who Pays First
When you have both Medicare and another form of health coverage, someone has to pay first. That determination — which insurer is primary and which is secondary — is called coordination of benefits, and getting it wrong can leave you personally responsible for medical bills that should have been covered.
The rules aren't complicated once you know them, but they're ruthlessly specific. The primary payer processes the claim and pays its share first. The secondary payer then covers some or all of the remaining balance, up to its own coverage limits. If the wrong payer is listed as primary, claims get denied, delayed, or underpaid.
The 20-Employee Rule
For people aged 65 and older with employer-sponsored group health coverage, one number determines everything: does the employer have 20 or more employees?
20 or more employees: The employer group health plan is primary. Medicare is secondary. You can safely delay Part B enrollment without a penalty, because you have qualifying active coverage. When claims come in, the employer plan processes them first, and Medicare picks up the rest.
Fewer than 20 employees: Medicare is primary. The employer plan is secondary. This means Medicare is expected to pay first — and if you haven't enrolled in Part B, there's no primary payer processing your outpatient claims. The employer plan, acting as secondary, may refuse to pay anything that Part B would have covered. The result: you're personally liable for the 80% that Part B normally handles.
This 20-employee threshold applies to the employer's total workforce, not just the employees on the health plan. If a company has 25 employees but only 8 are enrolled in the group health plan, the employer plan is still primary because the company employs 20 or more people.
Medicare Secondary Payer in Other Situations
The 20-employee rule covers the most common scenario, but Medicare Secondary Payer regulations extend to several other situations:
Disability coverage (under 65). If you're on Medicare due to disability and also have employer coverage through your own current employment (or your family member's), the employer must have 100 or more employees for the employer plan to be primary.
End-Stage Renal Disease (ESRD). Medicare becomes primary after a 30-month coordination period. During those first 30 months, the employer plan pays first.
Workers' compensation. For work-related injuries, workers' compensation is always primary. Medicare doesn't pay for services covered by a workers' comp claim.
Auto or liability insurance. If a car accident or other liability situation caused the medical need, the liability insurer or no-fault auto insurance pays first.
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What Happens When Coordination Goes Wrong
The most dangerous coordination error is also the most common: a person over 65 works for a small employer (fewer than 20 employees), assumes their employer plan is handling everything, and doesn't enroll in Part B.
Here's what that looks like in practice. You go to the doctor for an outpatient visit. The provider bills your employer plan. The employer plan checks Medicare's records and sees that Medicare should be primary for you — but you're not enrolled in Part B. The employer plan reduces its payment to what it would pay as a secondary payer, which might be 20% of the bill or less. Nobody is paying the primary share. You get the balance bill.
Meanwhile, every month that passes without Part B enrollment adds to your permanent late-enrollment penalty. Two years of delay means a 20% surcharge on your Part B premium for the rest of your life.
How to Check Your Coordination Status
Call the Benefits Coordination & Recovery Center (BCRC) at 1-855-798-2627. They maintain the Medicare Secondary Payer database and can tell you what coverage Medicare has on file for you and which insurer is listed as primary.
Review your Medicare Summary Notices (MSNs). These quarterly statements show how claims were processed and which payer was listed as primary.
Ask your employer's HR department for a copy of the group health plan's coordination of benefits provisions. The plan document specifies how it coordinates with Medicare.
Check your employer's size. Count all employees, not just those on the health plan. If it's borderline — the company had 22 employees last year but laid off 4 — the rule looks at whether the employer had 20 or more employees on 20 or more calendar weeks in the current or preceding year.
Fixing a Coordination Error
If you discover that Medicare should have been primary and you've been going without Part B, you need to act fast:
Determine your enrollment window. If you're still actively employed with the small employer, you're currently in your Special Enrollment Period and can sign up for Part B immediately using Form CMS-40B.
If active employment already ended, check whether you're still within the 8-month SEP window. If so, enroll immediately.
File for reconsideration if a penalty was assessed incorrectly. Use Form SSA-561-U2 with documentation showing your employment situation.
Contact the BCRC to update the Medicare Secondary Payer database with your correct coverage information.
For a complete walkthrough of the enrollment process, penalty calculations, and appeal procedures, the Medicare Late-Enrollment Penalties and Special Enrollment guide includes a 20-employee headcount audit worksheet and coordination of benefits verification checklist.
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