Medicare Secondary Payer (MSP): How to Get the Payment Order Right
When Medicare is not primary, billing it first is an instant denial — and getting MSP wrong is a compliance risk. Here are the common scenarios, the 20-employee rule, and how to bill secondary claims cleanly.
Medicare is not always first in line — and billing it as if it were is an instant denial and a compliance flag. Every practice that sees Medicare patients has to answer one question at registration: is Medicare primary here? Get it right and claims flow; get it wrong and you're reworking denials or, worse, drawing scrutiny.
What MSP is
Medicare Secondary Payer (MSP) rules cover every situation where another payer is responsible before Medicare. Federal law requires you to determine whether Medicare is primary for each service — it's not optional, and the responsibility sits with the billing provider.
Common scenarios
- Working aged (65+): covered by a group health plan through current employment — 20+ employees makes the group plan primary; fewer than 20 makes Medicare primary.
- Retiree coverage: Medicare is always primary; retiree plans pay secondary.
- Workers' compensation, liability, and no-fault/auto: those payers are primary for related care.
- Disability with a large group health plan, and ESRD have their own MSP rules.
The 20-employee line decides most MSP calls. Ask about current employment and plan size at check-in, every time.
The MSP questionnaire
The cleanest way to make the determination is to use the CMS MSP questionnaire (or one that asks the same questions) at registration and periodically thereafter. It captures current employment, other coverage, and accident/injury circumstances — exactly the facts that decide primacy. Document the answers in the patient account.
Billing secondary claims
When Medicare is secondary, bill the primary payer first, then submit to Medicare with the primary's adjudication — the EOB attached on paper, or the corresponding fields completed electronically. Remember the one-year filing limit from the date of service. This front-end diligence is the same discipline that prevents coordination-of-benefits denials and keeps eligibility verification clean.
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The bottom line
MSP comes down to one habit: determine Medicare's position before you bill. Use the questionnaire, apply the 20-employee rule, bill the primary first, and attach the EOB — and Medicare secondary claims stop bouncing. Start with a free billing audit.
Sources
Frequently asked questions
Most commonly when a beneficiary (or spouse) is still working and covered by a group health plan at an employer with 20 or more employees, or in situations involving workers’ compensation, liability, or no-fault insurance. In those cases the other plan pays first.
For beneficiaries 65+ covered by a group health plan through current employment: if the employer has fewer than 20 employees, Medicare is primary; if it has 20 or more, the group plan is primary and Medicare is secondary.
Yes. Medicare is always primary to retiree health coverage. Any plan provision attempting to make Medicare secondary for retirees is void under the MSP statute.
Bill the primary payer first, then submit to Medicare with the primary payer’s EOB information (attached on paper, or in the appropriate electronic fields). You have one year from the date of service to file.
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