A retired patient has Medicare and a supplemental plan through a former employer. Then she takes a part-time job, picks up her new employer's group coverage, and doesn't mention it at check-in. Medicare keeps paying as primary for months until a coordination of benefits review flags the claims and recoups the payments. The practice now has to rebill everything, in the right order, against deadlines that are already running. This article covers how to get payer order right the first time and how to file secondary claims that pay.

Determining Which Payer Is Primary

Coordination of benefits (COB) rules decide which plan pays first. The most common scenarios in specialty practices include:

• Medicare and employer coverage based on current employment. For patients 65 or older covered through their own or a spouse's current employment, the group health plan generally pays first when the employer has 20 or more employees. For patients under 65 who have Medicare due to disability, the threshold is generally 100 or more employees.

• Medicare and retiree coverage. Retiree plans generally pay after Medicare.

• End-stage renal disease. A group health plan pays primary during a coordination period that applies once the patient becomes eligible for Medicare based on ESRD.

• Dependent children with two parents' plans. Many plans use the "birthday rule," where the parent whose birthday falls earlier in the calendar year holds the primary plan. Divorce decrees and custody arrangements can change this.

• Medicaid. Medicaid is the payer of last resort and pays after every other source of coverage.

• Auto, workers' compensation, or liability. These often pay before health insurance for related injuries.

Payers apply their own COB rules too, including tie-breakers for situations the standard rules don't settle, so when the order isn't obvious, confirm with each payer and document the call.

Medicare Advantage adds its own wrinkle. A patient enrolled in a Medicare Advantage plan isn't billed to traditional Medicare at all, and Medigap policies don't pay secondary to Medicare Advantage. If a patient has moved to an Advantage plan and still carries an old supplement card, the practice needs to confirm which coverage is actually active before billing either one.

Collecting COB Information at Intake

Most COB problems start at registration. Patients don't volunteer that a spouse added them to a plan or that they returned to work. Staff need to ask directly.

A strong intake process includes:

• A Medicare Secondary Payer questionnaire for Medicare patients, completed at the first visit and updated periodically, as CMS expects providers to collect this information

• A direct question at every visit: "Has anything changed with your insurance, including any other coverage?"

• Scanning all cards, front and back, for every plan the patient has

• Recording subscriber details for each policy, including the policyholder's name, date of birth, and relationship to the patient

Eligibility responses sometimes indicate other coverage on file with the payer. Staff should check those indicators and resolve them before the claim goes out, since payers will deny when their records show another primary plan.

It helps to give front-desk staff a short reference card listing the common COB scenarios and the usual payer order for each. Registration staff rarely need to memorize the rules; they need to recognize when a situation is unusual and flag it for the billing team to confirm.

Filing the Secondary Claim

A secondary claim isn't a copy of the primary claim. It must carry what the primary payer did with each line.

On electronic claims, this means reporting the primary payer's paid amounts and adjustments, including the claim adjustment group and reason codes, in the coordination of benefits portion of the 837 transaction. Most practice management systems pull this from the primary ERA automatically, but only if the payment was posted correctly at line level. Posting a lump-sum payment without line detail is a common reason secondary claims fail.

Crossover claims. Medicare automatically forwards many claims to supplemental plans and Medicaid through its coordination of benefits agreement process. When a crossover is expected, check the Medicare remittance for crossover indicators before submitting the secondary claim manually. Sending both creates duplicate claims and confusion.

Timing. The secondary payer's filing window often runs from the date the primary processed the claim. Release secondary claims promptly after primary payment posts rather than letting them accumulate.

Some secondary payers still require a paper claim with the primary explanation of benefits attached. Keep a list of those payers so staff know which claims can't go out electronically, and scan the primary EOB into the patient account when it arrives so it's ready to attach.

For practices with a large Medicare population, this guide to Medicare billing fundamentals covers how Medicare's own processing affects downstream claims.

Common COB Denials and How to Resolve Them

A few denial patterns account for most secondary billing problems:

• Other coverage indicated (CARC 22). The payer believes another plan is primary. Confirm with the patient, update the payer order, and if needed ask the patient to update their information with the payer, since many plans require the member to complete a COB update.

• Primary payment information missing. The secondary claim went out without the primary adjudication data. Resubmit with complete line-level COB information.

• Recoupment after retroactive COB. A payer that paid as primary later determines it was secondary and takes the money back. Bill the correct primary promptly, using the recoupment notice as documentation if filing deadlines are an issue.

• Patient responsibility misbilled. The patient was billed for the primary's patient responsibility before the secondary processed. Wait for secondary adjudication before sending statements.

• Secondary payer requires prior authorization. Some secondary plans apply their own authorization rules even when the primary approved the service. Check secondary requirements during verification, not after the claim is denied.

Tracking these denials by reason each month shows whether problems are originating at intake or in payment posting.

When a pattern appears, fix it at the source. Repeated CARC 22 denials point to intake questions that aren't being asked. Repeated missing-data denials point to payment posting habits. Training the right team on the right step usually clears the backlog within a couple of billing cycles.

Billing the Patient After Both Payers

Once both payers process the claim, the remaining patient responsibility should be clear. Review it before the statement goes out. Certain combinations limit what the patient can be billed. For example, patients enrolled in the Qualified Medicare Beneficiary program can't be billed for Medicare deductibles, coinsurance, or copays, even when Medicaid pays little or nothing toward them.

Practices that find secondary claims consuming too much staff time sometimes turn to a billing partner that works secondary claims daily so the queue doesn't build up.

The Takeaway

Secondary insurance billing works when payer order is right, primary payment data is posted in detail, and secondary claims go out promptly. Ask about other coverage at every visit, use the Medicare questionnaire consistently, watch for crossovers, and track COB denials by cause. Those habits recover the second payment that many practices leave sitting in their A/R.