The one idea: If an employer has 20 or more employees, its group plan pays before Medicare. Get this backwards and Medicare can claw the money back years later.
Medicare’s rules about who pays first, the group plan or Medicare, aren’t just confusing. They can be expensive if you get them wrong.
The 20-employee threshold
Under Medicare Secondary Payer (MSP) “Working Aged” rules, Medicare is secondary (the group plan is primary) when the employer is “large.” Large means 20 or more employees on each working day in at least 20 calendar weeks in the current or prior year. The weeks don’t have to be consecutive, and part-time employees count as long as they’re on the rolls each working day of those weeks.
When the switch happens
If the employer met the test last year, the group plan is primary for the whole current year. If they cross 20 for the first time this year, the plan becomes primary once the 20th qualifying week is reached, and stays primary for the rest of this year and all of next year, even if headcount later dips.
Why carriers send the MSP questionnaire
Each year many carriers and TPAs ask employers to complete a Medicare Secondary Payer questionnaire. It isn’t busywork. They use it to determine who’s primary and avoid paying claims incorrectly. Accurate answers protect the employer from disputes and clawbacks later.
The clawback risk
If Medicare pays as primary when it should have been secondary, it can demand the money back, generally within three years of the date of service, longer if fraud or misrepresentation is suspected. On a fully insured plan that exposure can land on the carrier; on a self-funded plan, on the employer. (CMS: Group Health Plan Recovery)
Why it matters to you
This is a classic place where a broker adds real value. Help your group track weekly headcount (including part-timers), keep records of when the 20-week threshold is met, and complete the MSP questionnaire accurately, especially for self-funded groups where the recovery exposure is theirs.
Key takeaways
- 20+ employees (20 weeks, current or prior year) = group plan primary, Medicare secondary.
- Part-timers count; the weeks don’t have to be consecutive.
- Once you’re “large,” it sticks for the rest of the year and all of the next.
- Getting it wrong invites a Medicare clawback years later.
Check yourself
A company first hits 20 employees in March and stays at 20+ for 20 non-consecutive weeks in 2025. When does the group plan become primary, and for how long?
Starting with week 21 of 2025, and it stays primary for the rest of 2025 and all of 2026.