Wound Care Secondary Payer Billing: Medicare + Medigap

Wound Care Secondary Payer Billing: Medicare + Medigap + Commercial

Most wound care patients do not have a single payer. They have two, sometimes three. Medicare plus a Medigap supplement. Medicare plus Medicaid (dual eligible). Medicare plus an employer group health plan. Commercial primary plus a secondary commercial plan. Each combination has its own rules for which payer goes first, what the secondary payer owes, and how the claim gets there.

The practices that bill only the primary payer and write off the patient responsibility are leaving 15-30% of collectible revenue on the table. The 20% Medicare coinsurance on a $180 wound care visit is $36. Multiply that by 20 visits per week, and you are writing off $37,440 per year in secondary payer revenue that your patient's insurance would have covered.

Which Payer Is Primary?

Coordination of benefits (COB) determines which payer is responsible first. The primary payer processes the claim, pays its portion, and generates a remittance advice showing what it paid and what remains. The secondary payer then processes the balance.

Getting the sequence wrong causes denials on both sides. The secondary payer will deny if it receives a claim that should have gone to the primary first.

Medicare as Primary

Medicare is the primary payer in the most common scenario: a Medicare beneficiary age 65+ who is retired, has no employer coverage, and may have a Medigap supplement or Medicaid as secondary.

Medicare as Secondary (MSP Rules)

Medicare is the secondary payer when the beneficiary has coverage through one of these sources:

Employer group health plan (EGHP) — If the beneficiary or their spouse is actively employed and the employer has 20+ employees, the employer plan is primary. This applies regardless of age.
Large group health plan (LGHP) — For beneficiaries under 65 who qualify for Medicare due to disability, an employer plan from an employer with 100+ employees is primary.
Workers' compensation — If the wound is work-related, workers' comp is primary. Medicare does not pay for services covered by workers' comp.
Auto/liability insurance — If the wound resulted from an accident with liability coverage, that coverage is primary.
ESRD coordination period — For the first 30 months of Medicare eligibility due to end-stage renal disease, an employer group health plan is primary.

The MSP Questionnaire

Medicare requires providers to determine primary payer status at every visit — not just intake. Employment status changes. Insurance coverage changes. The MSP questionnaire (or its equivalent in your intake workflow) should be updated at least annually and whenever the patient reports a change.

Medigap (Medicare Supplement) Billing

Medigap plans are standardized by CMS into lettered plans (A through N). They are designed to cover some or all of the cost-sharing that Medicare beneficiaries owe after Medicare pays — deductibles, coinsurance, and copays.

Auto-Crossover

Most Medigap claims process automatically. When you submit a claim to Medicare and Medicare pays, the Medicare system checks whether the beneficiary has a Medigap plan enrolled in the Coordination of Benefits (COB) file. If so, Medicare automatically forwards ("crosses over") the claim to the Medigap insurer. You do not need to submit a separate claim.

For auto-crossover to work:

The Medigap plan must be registered in the COB file maintained by the Benefits Coordination & Recovery Center (BCRC)
Your practice must be enrolled as a participating provider with Medicare (accepting assignment)
The claim must be submitted electronically

If auto-crossover is working correctly, you will receive a payment from the Medigap plan within 2-4 weeks after the Medicare remittance, without submitting a separate claim.

When Auto-Crossover Fails

Auto-crossover does not work when:

The Medigap plan is not registered in the COB file (common with newer plans or patients who recently enrolled)
The claim was submitted on paper instead of electronically
The Medicare claim was adjusted or corrected after initial processing

When auto-crossover fails, you must submit a secondary claim manually. The secondary claim includes the Medicare remittance advice (ERA/EOB) showing what Medicare paid, the patient responsibility, and the reason codes. Most clearinghouses can submit secondary claims electronically using the 837P with the Medicare payment information populated in loop 2320.

What Medigap Covers for Wound Care

Medigap Plans C, D, F, G, and N cover Part B coinsurance (typically 20% of the Medicare-approved amount after deductible). For a wound care visit where Medicare pays $144 (80% of $180 allowed), the Medigap plan would cover the remaining $36 (20% coinsurance).

Plans K and L cover 50% and 75% of the coinsurance respectively, with an annual out-of-pocket limit. Plan N covers the coinsurance but may apply a copay for certain office visits.

Medicare + Medicaid (Dual Eligible)

Dual-eligible patients have both Medicare and Medicaid. Medicare is always primary. Medicaid is the payer of last resort.

How Billing Works

Submit the claim to Medicare first
Medicare pays its portion
The claim crosses over to Medicaid (most states have auto-crossover for dual eligibles)
Medicaid pays up to its fee schedule amount, minus what Medicare already paid

In most states, Medicaid will pay the Medicare coinsurance and deductible for dual-eligible beneficiaries, reducing patient out-of-pocket to zero. However, if Medicaid's fee schedule for the service is lower than Medicare's, Medicaid may pay less than the full coinsurance — or nothing at all if the Medicare payment already meets or exceeds Medicaid's rate.

Dual-Eligible Billing Rules

Never bill the patient for the difference between Medicare's approved amount and Medicaid's payment. Dual-eligible patients are protected from balance billing.
Submit to Medicare first, always. Submitting to Medicaid before Medicare results in a denial.
Check whether your state's Medicaid program has auto-crossover with Medicare. Most do. If not, submit secondary claims manually with the Medicare remittance attached.

Commercial Primary + Medicare Secondary

When a patient has commercial insurance as primary (typically through active employment), submit to the commercial plan first. After the commercial plan processes and pays, submit the balance to Medicare as secondary.

The Secondary Claim to Medicare

Medicare as secondary payer requires a claim that includes:

The primary payer's payment amount
The primary payer's allowed amount
The primary payer's adjustment reason codes
The primary payer's claim number

Medicare will process the claim and pay the lesser of:

The Medicare-approved amount minus the primary payer's payment, or
The Medicare coinsurance/deductible amount

If the primary commercial plan paid more than the Medicare-approved amount, Medicare pays nothing — the beneficiary has no Medicare liability on that claim.

Commercial Primary + Commercial Secondary

For patients with two commercial plans (e.g., coverage under their own employer and their spouse's employer), the Birthday Rule determines primary:

The plan of the person whose birthday (month and day, not year) comes first in the calendar year is primary
The other plan is secondary

If both policyholders share the same birthday, the plan that has been in effect longer is primary.

Submit to primary first, then submit the EOB and balance to secondary. The secondary plan will coordinate benefits according to its policy terms — some pay the remaining balance up to the allowed amount, others pay up to their own fee schedule.

Practical Workflow for Secondary Billing

At Intake

Verify all insurance coverage — primary, secondary, and tertiary
Confirm the COB sequence using MSP questionnaire for Medicare patients
Check whether secondary payer is enrolled in auto-crossover
Document all payer information including plan IDs, group numbers, and subscriber information

At Claim Submission

Submit to primary payer first
Wait for the primary remittance (ERA/EOB)
If auto-crossover is active, monitor for secondary payment
If no auto-crossover, submit secondary claim with primary payment information within 14 days of receiving the primary remittance
Post both payments and calculate any remaining patient responsibility

Common Secondary Billing Errors

Submitting to secondary before primary processes — Always wait for the primary remittance. The secondary payer needs the primary payment data to calculate its liability.
Not following up on auto-crossover — Auto-crossover can fail silently. If you don't see a secondary payment within 30 days of the primary remittance, investigate and submit manually if needed.
Ignoring the secondary altogether — This is the most costly error. The revenue is owed to you. The patient's insurance contract covers it. Failing to bill secondary means either the patient pays out of pocket unnecessarily or the practice absorbs the cost.
Wrong subscriber information on secondary — The secondary plan subscriber may be different from the patient (spouse's plan, parent's plan). Verify subscriber details separately from patient demographics.

The Revenue at Stake

For a practice billing $40,000 per month to Medicare with an average 20% coinsurance:

$8,000/month in coinsurance is billable to secondary payers
At a 75% secondary collection rate, that is $6,000/month in recovered revenue
Annual impact: $72,000 in revenue that was earned, is owed, and only requires a secondary claim to collect

Secondary payer billing is not optional revenue. It is earned revenue sitting in a queue, waiting for a claim that many practices never submit.

Key Takeaways

Secondary payer billing recovers the 20% Medicare coinsurance that most practices leave uncollected -- potentially $72,000 annually for a practice billing $40,000/month to Medicare
Medigap plans often auto-crossover from Medicare -- verify whether your clearinghouse supports automatic secondary submission to avoid manual claim creation
Always determine primary vs secondary payer using Medicare Secondary Payer rules before submitting -- incorrect ordering causes denials on both claims
Set up electronic secondary claim submission and build it into your standard billing workflow rather than treating it as a separate, optional process

For guidance on setting up electronic secondary claim submission, see our electronic billing guide.