Medicare Secondary Payer (MSP) Billing 2026

Who Pays First, and the Questions You Have to Ask Every Time

By Yagnesh Dave  ·  October 4, 2026  ·  12 min read

A practice bills 6,000 Medicare Part B claims a year. About 4% of those patients, 240 claims, turn out to have employer coverage that should have paid first. At an average of $110 a claim, Medicare paid $26,400 it shouldn't have, and every dollar is exposed to recoupment once a contractor notices.

Medicare Secondary Payer (MSP) rules decide who pays first when a patient has more than one coverage. They're simple in principle and messy in practice, because the answer depends on employer size, the patient's age or disability, a kidney diagnosis, an accident or a work injury, and all of it can change mid-year.

Here's who pays first, what you're required to ask, how Medicare pays when it's second, and what to do when the claim went to the wrong payer.

Medicare Secondary Payer Rules: Who Pays First

Medicare is the primary payer for most beneficiaries. It becomes secondary only in the situations the law lists, and each has its own test. Get the test wrong and either you bill the wrong payer first or you hold a claim that should have gone out.

Medicare is secondary for the working aged only when the employer meets the 20-employee test, which counts employees on each working day in 20 or more calendar weeks in the current or previous year. A patient at a 15-person firm with a group plan has Medicare as primary, even though they have employer coverage.

A spouse's employment can make the employer plan primary too. A 67-year-old retiree with Medicare and a 62-year-old spouse still working at a 200-person company may have the spouse's plan pay first, depending on how the patient is covered.

The MSP Questions You Have to Ask

CMS expects you to find out about other coverage and to bill the right payer first. The Medicare Secondary Payer Manual includes model questions for exactly this, and Chapter 3 treats them as the standard way to document that you asked.

The questions cover the situations in the table above. Is the patient being treated for a work injury or illness? Was there an accident, and could another insurer or a lawsuit be involved? Does the patient have group health coverage through their own or a spouse's employment? If so, how many employees does the employer have? Does the patient have ESRD, and if so, where are they in the 30-month coordination period?

Providers with online access can also check the MSP record in the eligibility transaction (HETS 270/271) or Medicare's common working file. CMS says that data should be viewed before the bill goes out, but having the data doesn't excuse you from asking the questions when something is unclear or has changed.

How Medicare Pays as the Secondary Payer

When Medicare pays second, it isn't simply the remaining balance. It runs three calculations and pays the lowest. The manual describes them in terms of the billed amount, the amount Medicare would have paid as primary, and the higher of Medicare's or the primary payer's allowed amount, each less the primary payment.

Take a $300 charge. Medicare would allow $200 and pay $160 as primary (80%). The primary payer allows $250 and pays $200. The three results are $160 for the normal Medicare payment, $100 for the billed amount minus the primary payment, and $50 for the higher allowed amount ($250) minus the primary payment. Medicare pays the lowest, $50.

That $50 may look odd next to a $160 primary payment, but it's how the formula works. What matters for billing is that the primary payer's explanation of benefits has to be right. If the claim doesn't include the primary payer's allowed amount, the payments and the adjustments, Medicare can't calculate the secondary payment correctly.

Insurance Type Codes and What Goes on the Claim

When you send a secondary claim, the claim tells Medicare why it's secondary. On the electronic claim, an insurance type code in the subscriber loop identifies the reason, and the primary payer's payment and adjustments go in the claim and line-level adjustment segments.

Use the code that matches the situation, not the one that gets the claim through. A claim sent as working aged when the real primary is workers' comp can be paid by the wrong payer and recouped later.

Accidents, Workers' Comp and Liability Claims

Accident cases are where Medicare is billed in error most often. If a patient's injury might be covered by workers' compensation, you have to check that first. Chapter 3 of the manual tells providers to find out whether benefits may be payable under workers' compensation, and in some cases a Medicare set-aside arrangement is in place that limits what Medicare will pay for related care.

For auto accidents, you bill the no-fault or medical payments coverage first. If it doesn't pay, you can bill Medicare, but you'll need to show you tried. For liability cases, Medicare can make a conditional payment when the liability claim is pending and recover it from the settlement. Federal regulation 42 CFR 411.54 limits what you can charge a beneficiary once a liability payment has been received.

Conditional payment exists so providers aren't left waiting. Under 42 CFR 411.21, prompt payment by a primary payer means within 120 days after it receives the claim, with a separate rule for liability insurers. When it hasn't, Medicare can pay conditionally and recover from the primary payer or the beneficiary later.

When Medicare Paid First by Mistake

It happens in every practice. A patient's employer coverage is found two months after Medicare paid as primary, or the MSP record updates and a claim you billed correctly is now wrong.

The manual's rules for duplicate payments and incorrect primary payments tell you what to do. If you've received payment from Medicare and another payer for the same service, you generally have to refund the Medicare overpayment. Under the 60-day overpayment rule, the clock starts when you knowingly identify it, and you need to look back six years for similar claims.

Work these in order. Stop billing Medicare as primary for that patient immediately, bill the true primary, refund or adjust the Medicare payment through your Medicare Administrative Contractor, and write down when and how you found it. Then ask whether the same thing happened on other claims for the same employer or the same patient.

Our guide to the Medicare 60-day overpayment rule walks through the investigation and refund steps in detail.

MSP Audit and Recovery Exposure

MSP errors are among the easiest overpayments for CMS to find, because the data sits in the same systems. CMS's fiscal year 2025 data puts the Part B provider improper payment rate at 8.44%, or $9.62 billion, and a share of that is Medicare paying when another payer should have.

Recovery works on two tracks. CMS contractors recover conditional payments from primary payers and beneficiaries, and they can recover from a provider that was paid in error. On the insurer side, the Section 111 reporting program requires insurers and self-insured plans to report coverage to CMS, and CMS has a civil money penalty rule for those that don't. Better reporting makes MSP data more accurate, which means old billing mistakes get easier to spot.

Medicare contractors can reopen a claim within 1 year for any reason and within 4 years for good cause. Keep the MSP questionnaire, the eligibility response and your billing notes for 10 years, so you can show what you knew at the time of billing.

Frequently Asked Questions About Medicare Secondary Payer Billing

When is Medicare the secondary payer?

Medicare is secondary when another coverage is primary. That includes an employer group health plan for working people 65 or older at employers with 20 or more employees, a large group health plan for disabled people at employers with 100 or more employees, and workers' compensation, liability and no-fault insurance.

How long does the ESRD coordination period last?

Thirty months. During that period an employer group health plan stays primary for a person with end-stage renal disease, and Medicare pays second.

What if the other insurer doesn't pay promptly?

Medicare can make a conditional payment so you aren't waiting, and then recover from the primary payer. Under 42 CFR 411.21, prompt payment from a primary payer means within 120 days after it receives the claim, with a separate rule for liability insurers.

How long should I keep MSP information?

CMS recommends keeping MSP information for 10 years. It's the record that shows you asked the right questions if Medicare questions a claim later.