Coordination of Benefits: How Employer Plans and Medicare Decide Who Pays First This is coordination of benefits, or COB. It's the mechanism by which two insurance plans communicate and divide responsibility for a claim. And while the concept sounds straightforward (one plan pays first, the other pays what's left), the execution is remarkably complex. Claims get routed incorrectly. Plans don't communicate. Carriers disagree on who's primary. And when the coordination is wrong, someone, usually your plan, ends up paying when it shouldn't have, or not paying when it should.
For employers managing Medicare-eligible employees , understanding COB isn't optional. It's a compliance requirement. Here's what you need to know about who pays first, how claims move between payers, and what happens when coordination goes wrong.
Who Pays First: The MSP Rules for Employer Medicare The Medicare Secondary Payer (MSP) statute determines the order of payment. Under MSP, Medicare is sometimes primary , and sometimes it's secondary. The determination depends on the employee's situation.
For most active employees aged 65 and older who are covered by their employer's group health plan, the group plan is primary. Medicare is secondary. This is true regardless of whether the employee is full-time, part-time, or in any other employment status, as long as they're receiving active coverage through the employer. The employer group plan processes the claim first. It calculates its benefit and pays. Then the claim moves to Medicare, which looks at what the group plan paid and decides whether to pay additional benefits.
But there are important exceptions. If an employer has fewer than 20 employees , the coordination flips: Medicare becomes primary, and the group plan becomes secondary. If an employee is disabled (under 65) and on Medicare, and their employer has fewer than 100 employees, Medicare is primary. If an employee is on Medicare due to end-stage renal disease (ESRD), the coordination rules change depending on whether they have employer coverage and whether the employer is large or small. Get the size threshold wrong or misidentify the employee's status, and the primary/secondary order is reversed.
Ohio employers navigating Medicare coordination should pay particular attention to these thresholds. Many mid-size employers in Ohio Medicare markets fall right around these cutoffs, and the consequences of getting it wrong aren't theoretical.
Crossover Claims and the Claims Pipeline When a claim goes to the primary payer first and then to the secondary payer, that's called a "crossover claim" or sometimes a "crossover." The primary payer processes and pays. It then sends electronic information about that payment to the secondary payer, including what was paid, what the allowed amount was, and what the patient's liability is. The secondary payer uses that information to determine its own payment obligation.
But the pipeline can get clogged. Sometimes the claim doesn't reach the secondary payer at all because it gets lost in the carrier's electronic system. Sometimes the information sent to the secondary payer is incomplete or inaccurate. The primary payer might not send the deductible amount correctly, or the patient's coinsurance is miscalculated. When the secondary payer receives bad or incomplete information, it makes a payment decision based on incomplete data. And now you have a claim where the total paid out (primary plus secondary) exceeds what the actual medical bill was, or doesn't cover the patient's full obligation.
This is why some claims take a very long time to fully process. There's a conversation happening between payers, back and forth, to make sure the coordination is right before the claim is finally closed out and the patient's liability is settled.
What Happens When COB Is Wrong When coordination of benefits is wrong, whether because the primary/secondary order was reversed, or because claims weren't routed correctly, or because incomplete information was sent between payers, the employer plan usually ends up holding the bag.
Most commonly, the employer plan pays as secondary when it should have paid as primary. This happens when a claim is routed to Medicare first, Medicare pays based on its rules, and then by the time the claim reaches the employer plan, the employer plan assumes Medicare was primary and limits its payment accordingly. Or the employer plan is aware of Medicare but miscalculates what Medicare's payment obligation should have been, and pays too much as a result.
Another scenario: an employer plan fails to file the claim with Medicare altogether. This happens with some smaller plans or when coordination processes aren't well-established. The employer plan pays the claim in full, thinking that's the end of it. But Medicare later audits and determines it should have been primary, and now the employer plan owes recovery money because it paid when it shouldn't have.
The fix, when COB is discovered to be wrong, is usually a claims correction. The primary payer recalculates based on correct coordination, adjusts its payment, and sends corrected information to the secondary payer. But claims corrections can take months, and in the meantime, the patient may get bills for amounts they thought were covered, or they may see claims denied after they thought they were approved.
What Your Plan's COB Provisions Should Address Your group health plan's coordination of benefits provision is the legal foundation for how claims are processed in the presence of other coverage. That provision should clearly state: which payers are considered primary and which are secondary, whether your plan will pay if another plan is primary, and how your plan handles disputes about coordination.
A good COB provision also addresses the "benefits-based" approach, where the secondary payer is not required to pay more than what the primary payer should have paid under its own benefit rules. This prevents the total payment from exceeding the actual bill. It should also spell out what happens if the primary payer doesn't pay as expected, and whether your plan will pay the full benefit, a reduced benefit, or nothing while waiting for the primary payer to respond.
From an operations perspective, your plan should have clear procedures for identifying Medicare-eligible participants, determining primary/secondary status, and ensuring claims are filed with both payers in the correct order. This is often handled by your plan administrator or third-party administrator (TPA), but it's worth auditing periodically to make sure the process is actually happening. If your plan isn't systematically filing claims with Medicare when required, you have a coordination leak.
Coordination of benefits is invisible when it works. But when it breaks, it breaks publicly, usually when the patient gets a bill they didn't expect, or a claim is delayed for months. Know your plan's COB rules inside and out, because they're the difference between smooth claims processing and unnecessary compliance headaches.
Medicare for employers isn't just about enrollment. It's about making sure every claim touching Medicare is coordinated correctly, every time.
Next issue in the Exact Benefits Newsletter: The Most Common Medicare Mistakes Employers Make. A plain-English list of what goes wrong and how to avoid it.
