You bought stop loss insurance to protect your company from a catastrophic claim. You paid the premium, set the attachment point, and signed the contract. Then a $200,000 claim lands on your desk. Your TPA pays it. You expect a nice reimbursement check.
Instead, you get a denial letter. The reason: "Amount exceeds reasonable and customary charges." The stop loss carrier only sent $50,000, not the $160,000 you were banking on.
The root cause was a data translation failure, not a coverage dispute or a medical necessity question. And it happens far more often than most employers realize.
The Two Systems That Never Talk to Each Other
When a large claim is processed, two different software systems touch the same data, but they don't speak the same language.
- The TPA's system is built for benefit plan administration. It applies the Summary Plan Description logic: deductibles, coinsurance, out-of-network penalties, coordination of benefits. Its job is simple: pay the provider correctly per the plan document.
- The stop loss carrier's system is built for policy contract administration. It takes the raw claim feed (usually an 837 file) and re-adjudicates the entire claim against its own interpretation of "eligible expenses." It doesn't trust the TPA's work at all.
These two systems operate in parallel, but they rarely reconcile. And the employer, the one paying for both, ends up stuck in the middle, holding the bag.
The Silent Re-Percentage Problem
The most dangerous and least discussed issue in stop loss claims is the way the carrier's system re-processes the claim as if it were running your actual benefit plan. But it doesn't have your plan design loaded in real time. Instead, it uses a generic UCR (Usual, Customary, and Reasonable) database from a third party like FAIR Health.
If that database is six months old, or uses a different geographic region, or fails to recognize your PPO network discount, the system flags the claim as "excessive." It then reduces the allowable amount and pays you less. You have no way to see the carrier's internal UCR table. No easy way to challenge it. And most employers never even know it happened. They just see a smaller check and assume that's the system working as designed.
A real-world example: Your plan covers a $100,000 hospital stay, and your negotiated network rate drops the allowed amount to $60,000. The TPA pays $60,000. The stop loss carrier's system, lacking access to your negotiated contract, pulls a UCR value of $45,000 for that stay in your region. It then denies $15,000 of what the TPA already paid. You lose the difference because the data didn't flow, not because the care was unnecessary.
The Technical Denials That Have Nothing to Do with Health
Stop loss carriers routinely deny or reduce claims for reasons that have zero clinical basis. These are system mismatches, not coverage disputes. And they're maddeningly common.
- Revenue code vs. CPT code: The TPA processes a facility fee under Revenue Code 0450 (Emergency Room). The carrier's system looks for a specific CPT code. If the CPT code is missing, the carrier flags that claim line as non-eligible and holds up the entire claim.
- Bundled payment vs. line-item adjudication: Your plan has a bundled payment with a Center of Excellence, say, $50,000 for a knee replacement. The TPA processes a single lump sum. The carrier's system splits it into anesthesia, surgeon, implant, and facility fees. It can't match the lump sum to its line-item structure, so it kicks out for manual review. That manual review can take weeks or months.
- Discount degradation: The TPA pays $40,000 on a $100,000 claim because your PPO network negotiated a 60% discount. The carrier's system sees the $100,000 as the "allowable amount" but doesn't recognize the network contract. It applies its own lower discount, say 30%, and calculates reimbursement on $70,000 instead of $40,000. You lose the value of the network you paid for.
Pharmacy Claims Run Through a Third System
Pharmacy claims move through a third system entirely: the pharmacy benefit manager, or PBM. The PBM adjudicates the prescription at the point of sale, applies your formulary and rebate terms, and reports the result to the TPA, which the stop loss carrier then re-adjudicates against its own rules. The same translation gap applies, and the dollars at stake are climbing.
Specialty drugs are now the fastest-growing stop loss exposure. In NFP's 2026 employer survey, 51% of employers cited GLP-1s as the top driver of rising prescription drug costs, and nearly half of large employers covered them in 2025. A month of a GLP-1 runs $1,000 to $1,350 at list price before plan discounts. When a member on one of these drugs crosses the specific deductible, the carrier does more than re-price the claim. It often applies a laser: a higher attachment point set on that one individual, sometimes two to three times the standard amount.
Lasers are a standard contract provision, and they land most often on people taking specialty medications. The data problem compounds from there. If the carrier's pharmacy feed doesn't match the PBM's adjudication record, a claim that should have counted toward the laser threshold can get dropped or double-counted. Whatever feed standards you demand for medical claims, demand them for the PBM's pharmacy feed too, and confirm the carrier ingests that feed unchanged.
Aggregate Stop Loss and the Incurred vs. Paid Date Mismatch
Most conversations focus on specific stop loss (claims over the attachment point). But aggregate stop loss is where the system friction really stings.
The core problem: stop loss eligibility depends on two dates, and each system emphasizes a different one. The TPA reports claims on an incurred-date basis, the way benefit plan liabilities are booked. The stop loss contract reimburses a claim only if it is incurred and paid within the contract period, typically a 12/12 policy or a 12/15 policy with a 90-day run-out. When a claim straddles the policy boundary, the two books drift apart.
A claim is incurred in December 2025 but paid in January 2026. The plan ledger books it in 2025. The stop loss carrier, applying the contract's incurred-and-paid window, books it in 2026. If your aggregate pool tracks incurred dates but reimbursement settles on paid dates, the two numbers will never match. You may think you stayed under the pool, and the carrier may say you exceeded it. The contract defines the rule; the two ledgers just apply different date conventions.
The fix is a quarterly reconciliation of incurred and paid dates for claims near the policy boundary. Most contracts include a run-out period, but few TPAs or carriers automate the reconciliation. So employers overpay for aggregate claims that should have landed in a prior year, or they miss reimbursement entirely.
What Best-in-Class Employers Should Demand
You can't fix the entire industry overnight. But you can protect your own organization. Four demands matter most:
- Require a standardized claim feed. Demand that your TPA send the raw 837 data, the same claim file it receives from providers, directly to the stop loss carrier. No re-pricing. No summarization. The carrier should ingest the exact same data the TPA used.
- Demand real-time benefit plan mirroring. The stop loss carrier's system should have an API connection to your benefit plan document, not a PDF scanned six months ago. The allowed amount and contractual obligations should match automatically.
- Implement pre-adjudication validation. Before a claim reaches a human adjuster, a machine-to-machine check should confirm that the TPA's payment aligns with the carrier's approved UCR database. If it does, auto-approve. If not, escalate.
- Audit your run-out period for aggregate claims. Run a reconciliation of incurred dates versus paid dates for all claims near the policy boundary. Do this quarterly. Don't wait until year-end to find a mismatch.
All four are contract terms, and they are negotiated at renewal. If your broker won't push for them, find one who will.
The Bottom Line
Stop loss insurance is a critical risk management tool. But it only works when the systems behind it work together. Today, they don't.
Lower premiums and higher attachment points have limits. The next frontier for stop loss is a unified claims data standard that ensures the system pays what the contract promises.
Until that day arrives, the costliest claim you process may be the one that slips into the data gap and goes unnoticed, rather than the one that clearly exceeds your attachment point.