How to Bill Medicare as a Secondary Payer

A claim sent to Medicare in the wrong order, or without the primary payer’s explanation of benefits (EOB), is likely to be delayed or rejected. This guide walks through the sequence a biller follows when Medicare pays second: verify other coverage, bill the primary payer, check the EOB, and submit to Medicare for Part A or Part B.

What Medicare Secondary Payer Means and Why Billing Order Matters

Medicare Secondary Payer describes a situation where another insurer or entity has responsibility to pay before Medicare does. Anyone billing Medicare is expected to confirm the order of payment before a claim goes out, because the order determines who receives the first bill.

One caution up front: the fact that a patient has other insurance does not, by itself, make Medicare the secondary payer. Age, disability, end-stage renal disease (ESRD), employment status, and the type of coverage all affect the order. Confirm it for each patient rather than assuming.

The details in this guide focus on Original Medicare (Part A and Part B). Patients enrolled in Medicare Advantage or similar plans should have their plan’s billing process confirmed separately.

Step 1: Screen Every Patient for Other Coverage

Start at the front desk. Ask each patient at the time of service whether they have other coverage that could pay before Medicare. CMS directs providers to check for current or spouse/family employer-based insurance, workers’ compensation, and auto or liability insurance. Part B providers are expected to verify insurance information at the visit.

Front-desk screening checklist

• Does the patient, or a spouse or family member, have employer-based health coverage that is current?
• Is the visit related to a work injury that may fall under workers’ compensation?
• Is the visit related to an auto accident or another liability situation?
• Has any coverage changed since the last visit?
• Are the answers and the insurance details recorded in the patient’s account?

Medicare.gov offers a plain-language overview of how Medicare works with other insurance and lists who to contact when a patient’s coverage changes.

Step 2: Bill the Primary Payer First

When Medicare is secondary, submit the claim to the primary insurer first and wait for it to process the claim. What happens next depends on the result:

• Primary pays in full: generally, no Medicare claim is needed.
• Primary pays only part, or denies: move on to Medicare, using the primary payer’s EOB.
• More than one primary payer: an EOB from each insurer may be required.

Some situations need extra care. Claims related to an accident or workers’ compensation should go to that insurer first, and where an employer group plan has priority, that plan may also need to be billed first.

Step 3: Read the Primary Payer’s EOB Before You Submit

Do not attach the EOB without reading it. An EOB that is missing a required element may delay processing, so check each item below before the claim leaves your office.

EOB element	Why Medicare needs it
Primary payer’s name and address	Identifies which insurer processed the claim first
Subscriber’s name and policy number	Links the EOB to the right coverage
Provider	Confirms who rendered the service
Itemized charges	Shows what was billed and how each line was handled
Denial and payment code explanations	Explains why each line was paid, reduced, or denied
Dates of service	Matches the EOB to the claim being submitted

If a code explanation or charge detail is missing, ask the primary payer for a complete copy before submitting to Medicare.

Step 4: Submit the Claim to Medicare (Part B)

Once the primary payer has processed the claim and did not pay in full, submit to Medicare in one of two ways.

• Paper: use the CMS-1500, enter the primary payer’s name and policy number, and attach a copy of the primary payer’s EOB.
• Electronic: include the primary payer’s payment information in the fields, loops, and segments required for processing.

The amount billed to Medicare should equal the amount you billed the primary payer. Billing Medicare only for the balance the primary left unpaid differs from First Coast Service Options (FCSO) guidance.

Worked example (illustrative numbers only)

Suppose you bill the primary insurer $200 and it pays $120. The claim you send to Medicare still shows the $200 charge, with the primary payer’s EOB attached, rather than a separate $80 claim. Medicare then reviews the claim under its own coverage rules, so what it pays is not necessarily the $80 difference.

Step 5: Submit the Claim to Medicare (Part A Institutional)

Institutional providers submit MSP information differently. For Part A, the primary payer details go in through condition codes, occurrence codes, and related data elements on the claim.

• Electronic routes: the 837I or FISS/DDE.
• Paper: the CMS-1450 (UB-04), where the requirements for paper submission are met.
• Supporting document: paper claims include the primary payer’s remittance advice (RA) or EOB.

Part A vs. Part B at a Glance

Item	Part B (professional)	Part A (institutional)
Paper claim form	CMS-1500	CMS-1450 (UB-04), where requirements are met
Electronic route	Electronic claim with the required fields, loops, and segments	837I or FISS/DDE
Where MSP data goes	Primary payer name and policy number on paper; MSP payment information in required electronic fields	Condition codes, occurrence codes, and related elements
Supporting document on paper	Copy of the primary payer’s EOB	Primary payer’s RA or EOB

Timely Filing: Why You Cannot Wait on Coverage Questions

Standard Medicare timely-filing limits apply to MSP claims. FCSO guidance states that misidentifying a patient’s insurance does not extend the filing deadline, so an unresolved coverage question does not buy extra time.

One practical suggestion: record the date you billed each primary payer and the date it responded, so you can see early when a claim is approaching the filing limit.

When the Primary Payer Is Slow: Conditional Payment Requests

A conditional payment request is a separate path from ordinary secondary billing. It applies when another insurer is responsible but is not expected to pay promptly, and 120 days have passed. Medicare may then make a conditional payment, which can be recovered later if the primary payer pays.

The 120 days is the benchmark for conditional payment requests only. It is not a general waiting period that applies to every MSP claim.

Exceptions apply. Claims related to accidents or workers’ compensation go to that insurer first, and employer group plan situations may require billing the primary plan first.

After Submission: Reading the Remittance Advice and Billing the Patient

After Medicare processes the claim as secondary payer, the Medicare remittance advice shows what may be collected from the patient. Use it, rather than the primary payer’s EOB, to determine the patient’s responsibility.

An EOB does not guarantee that Medicare pays the full remaining balance. Payment depends on Medicare’s own coverage review. Medicare.gov also notes that patients can be responsible for a balance in some cases.

If a claim is denied or rejected, check two things first: whether the payment order is correct, and whether the EOB is complete.

Quick Pre-Submission Checklist and Common Mistakes

Checklist

• Other coverage was screened and the payment order confirmed.
• The primary payer was billed first and has processed the claim.
• The EOB is complete, and every primary payer’s EOB is included if there is more than one.
• The billed amount matches what was billed to the primary payer.
• The right form or electronic data is used for Part A or Part B.
• The claim is within standard timely-filing limits.

Common mistakes

• Billing Medicare only for the leftover balance.
• Leaving out an EOB when a second payer is involved.
• Assuming any other insurance makes Medicare secondary.
• Treating 120 days as a standard wait for every claim.

Frequently Asked Questions

How do I know if Medicare is secondary?

Screen the patient for other coverage and confirm the payment order. Other insurance alone is not enough, because age, disability, ESRD, employment status, and coverage type all affect the order.

What if the primary payer denies the claim?

If the primary payer denies or pays only part of the claim, submit to Medicare with the primary payer’s EOB showing the denial or payment and its code explanations.

Can I bill Medicare before the primary pays?

Generally, no. Bill the primary payer first and wait for it to process the claim. The exception is a conditional payment request, which applies only when another insurer is responsible but is not expected to pay promptly and 120 days have passed.

Does this apply to Medicare Advantage patients?

This guide focuses on Original Medicare Part A and Part B. If a patient is enrolled in Medicare Advantage or a similar plan, confirm that plan’s billing process.

Where do I find what to collect from the patient?

Check the Medicare remittance advice. It shows what may be collected from the patient after Medicare pays as the secondary payer.

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