Look at the patient AR for a practice with an older population and you will find balances that are the Medicare coinsurance on visits from five or six months ago. The patient has a Medigap plan. Medicare paid. The Medigap plan never did, because the claim never reached it, and the balance rolled to the patient, who is now receiving statements for money her supplemental plan would have paid if anyone had asked.
Medicare's crossover process is one of the quiet efficiencies of the system: when it works, the secondary claim is sent for you, the supplemental pays, and nobody in the practice touches it. When it fails, nothing announces the failure. The claim simply never appears at the secondary, and the practice discovers it in patient AR or not at all.
Key takeaways
- Medicare forwards adjudicated claims to supplemental insurers through the Coordination of Benefits Agreement program, either from coverage on Medicare's file or from the Medigap identifier the practice puts on the claim.
- Remark codes MA18 and N89 on the Medicare remittance mean the claim crossed. No crossover remark means it did not, and the secondary needs a manual claim.
- Hold secondary claims when MA18 or N89 is present. Billing both ways creates duplicates and recoupments and hides the real failures.
- A manual secondary claim must carry Medicare's line-level adjudication, which is why lump-sum posting produces claims the secondary cannot read.
- Most failures are data problems that repeat: fix the Medigap identifier, the demographics or the plan type at registration so the next claim crosses.
How crossover works
Medicare's Coordination of Benefits Agreement program, administered through the Benefits Coordination and Recovery Center, lets supplemental insurers (Medigap plans, employer retiree plans, state Medicaid programs and others) receive Medicare's adjudicated claim data automatically. When Medicare processes a claim for a patient whose supplemental coverage is on file with Medicare, or whose Medigap plan is identified on the claim itself, Medicare sends the claim and its payment information to the supplemental insurer after adjudication. The insurer then pays its share, typically the Part B deductible and coinsurance, according to the plan.
There are two paths. Eligibility-based crossover uses the supplemental coverage recorded in Medicare's files, which the insurer supplies. Claim-based Medigap crossover uses the Medigap insurer's identifier that the practice places on the claim (loop 2330B on the 837P; item 9d on the CMS-1500) along with the patient's Medigap policy number, and requires the patient to have assigned Medigap benefits to the provider.
The timing is worth knowing. Medicare forwards the claim after it finalizes, so the supplemental insurer usually receives it within a few days of the Medicare remittance and pays within its own processing time, commonly two to four weeks. A secondary payment that has not arrived 45 days after Medicare's remittance is late enough to investigate.
How to tell whether a claim crossed
The Medicare remittance tells you. Look for these remark codes on each claim:
| Remark code | Meaning | What to do |
|---|---|---|
| MA18 | The claim information is also being forwarded to the patient's supplemental insurer | Wait for the supplemental remittance; do not bill the secondary manually |
| N89 | Payment information has been forwarded to more than one other payer | Same; expect more than one secondary remittance |
| No crossover remark | Medicare did not forward the claim | Bill the secondary manually after the primary remittance posts |
The practice management system should set a secondary claim to hold when MA18 or N89 is present and release it to a manual queue when they are absent. Practices that do this by hand can add the remark code to the posting screen and train the poster to look for it. The failure mode we see most is the opposite of the crossover failing: the practice bills the secondary manually on every claim, the crossover also arrives, and the secondary denies the manual claim as a duplicate or, worse, pays twice and later recoups.
Run one report a month to catch the other direction: claims with MA18 on the Medicare remittance and no secondary payment posted after 45 days. That list is short in a healthy practice and it is the list of crossovers that were sent but not paid, which is a different problem (usually the supplemental insurer has a coverage or demographic mismatch) and a different phone call.
Why crossovers fail
- The supplemental coverage is not on Medicare's file. The patient has a Medigap plan, but the insurer has not reported the coverage to Medicare, or reported it with a different name or date of birth than Medicare has. Nothing crosses.
- The Medigap identifier on the claim is wrong or missing. Claim-based crossover depends on the correct insurer identifier and policy number. A practice that scans the Medigap card but never enters the identifier will see no crossover for that patient.
- Medicare denied the claim. Denied claims generally do not cross, though some supplemental insurers request them. If Medicare pays nothing, the secondary usually needs a manual claim with the denial information, and may or may not pay.
- The patient has Medicare Advantage. There is no crossover; the Advantage plan is the primary and only payer for covered services. A Medigap policy cannot be used alongside Medicare Advantage. A claim sent to Medicare for an Advantage member is denied CO-109, and the practice's real problem is registration.
- Medicaid as secondary. States receive crossovers through their own agreements, and each has its own rules for processing them. Many pay the lesser of the Medicare cost sharing or the difference between the Medicaid rate and Medicare's payment, which is often little or nothing. Patients who are Qualified Medicare Beneficiaries cannot be billed for Medicare cost sharing at all, whatever Medicaid pays.
How to bill the secondary manually
When the crossover did not happen, the secondary needs a claim that carries Medicare's adjudication. Electronically, that means the 837P with the Medicare payment and adjustment information at the line level: the amount Medicare paid, the adjustment group and reason codes (the CAS segments) for the deductible, coinsurance and contractual amounts, and the adjudication date. Most practice management systems build this from the posted Medicare remittance, which is why accurate line-level posting matters; a remittance posted as a lump sum produces a secondary claim the payer cannot read. On paper, the secondary claim is the CMS-1500 with the primary insurance identified in items 9 through 9d or 11 through 11d as the payer instructs, and Medicare's remittance attached.
Timing matters. Many supplemental insurers will not process a manual secondary claim until a set period after Medicare's adjudication, commonly 30 days, so that the crossover has a chance to arrive first. Sending earlier produces duplicate denials. Secondary claims also have their own timely filing clocks, often measured from the primary remittance date, so the manual queue must be worked monthly, not annually.
A line-level example makes the point. Medicare allows $130 on a 99214, applies $50 to the deductible, pays 80 percent of the remaining $80, which is $64, and leaves $16 coinsurance. The secondary claim for that line must show the $64 payment, a PR-1 adjustment of $50, a PR-2 adjustment of $16 and a CO-45 adjustment for the difference between the charge and the allowed amount. A Medigap plan that receives those four figures pays the $66 the patient would otherwise owe. One that receives "Medicare paid $64" and nothing else rejects the claim for missing adjudication information, and the balance goes back to patient AR.
Fixing the crossover for next time
A failed crossover is usually a data problem that will fail again. For eligibility-based failures, the patient contacts their Medigap insurer to confirm the coverage is reported to Medicare with matching demographics; the practice cannot do this for them but can tell them exactly what to ask. For claim-based failures, the practice enters the Medigap identifier and policy number in the patient's insurance record so every future claim carries it. For Medicare Advantage confusion, the front desk confirms plan type at every visit. Keep a short list of patients whose crossovers failed and check the next remittance for MA18.
A worked example
An internal medicine practice with four physicians found $38,000 in patient AR that was Medicare coinsurance for patients with Medigap plans on file. On review, none of the Medicare remittances carried MA18 for those patients. The cause was a system conversion two years earlier that had dropped the Medigap identifier field for about 300 patients. The practice rebuilt the field from the scanned cards, billed the secondaries manually with Medicare's line-level adjudication attached, and recovered most of the balances that were still within the secondary plans' filing limits. The claims that had passed the limits were written off with the cause documented. The following month, MA18 appeared on those patients' remittances again.
Questions we hear
Medicare paid, the secondary paid, and the patient still has a balance. Why?
Usually because the secondary is not a full Medigap plan and pays only part of the cost sharing, or because the service is one the supplemental plan does not cover. Check the secondary's remittance codes before billing the patient, and explain the plan's limits to the patient rather than sending a bare statement.
Should we bill the secondary manually on every claim to be safe?
No. Bill manually only when the Medicare remittance shows no crossover remark. Billing both ways creates duplicates and recoupments and hides the real failures.
Who owns this?
The payment poster owns reading MA18 and N89 and routing claims to the manual queue. The follow-up biller owns the queue and the 30-day timing. The front desk owns Medigap identifiers and plan type at registration. If secondary balances are sitting in your patient AR, this is one of the first things a Revelrex RCM audit checks, and the secondary queue is a standard part of the billing service.
What to do this week
- Pull patient AR for Medicare patients with a secondary plan on file and check the Medicare remittances for MA18 or N89.
- Find out whether your practice management system holds secondaries automatically when a crossover remark is present, and turn that on if it exists.
- Check that the Medigap identifier and policy number fields are populated for every patient with a Medigap card scanned.
- Build the two monthly reports: no crossover remark and no manual secondary sent; MA18 present and no secondary payment after 45 days.
- Confirm that Medicare remittances are posted at the line level, with deductible, coinsurance and contractual amounts separated.
