A denied claim announces itself. An underpaid claim does not. The remittance arrives, the payment posts, the claim closes as paid, and the shortfall — a few dollars on one line, repeated across hundreds of lines — stays hidden inside an adjustment no one questioned. By the time a practice notices that a payer’s payments “feel low,” the pattern may have been running for months.
Spot checks will not fix that. The method here catches what occasional checks miss: build an expected-payment model from your own contracts, compare expected against paid on every remittance line, set a variance threshold for what gets worked vs. watched, and dispute patterns — not just single claims — with a verifiable packet. General underpayment recovery is covered elsewhere; this is the detection method underneath it.
Why paid claims need checking at all
An underpayment is specific: the payer paid less than the provider contract requires for that service, after patient responsibility is correctly separated. It is not the difference between the billed charge and the payment — that difference is mostly the contractual adjustment every in-network claim carries. It is not a denial, because something was paid. And it is not established by looking at the paid amount alone, which may be exactly right once deductible, copayment, or coinsurance is accounted for.
Three amounts must be separated on every line: the allowed amount the payer applied, the contracted rate expected for that payer, plan, code, modifier, place of service, and date of service, and the split between payer payment and patient responsibility. If the allowed amount matches the contract and the rest was correctly assigned to the patient, there is no underpayment, however small the check looks.
Posting accuracy matters too: loose posting corrupts any variance report. Our guides to EOB vs. ERA and payment reconciliation in medical billing cover reading and reconciling remittance data line by line.
Step 1: Load the contracts into an expected-payment model
For each payer contract, extract and record:
- The contracted rate or rate method for each CPT/HCPCS code, including how it is derived if expressed as a percentage of a fee schedule.
- How modifiers, multiple procedures, units, place of service, and provider type change the expected amount, where the contract addresses them.
- Effective and termination dates, including amendments and updated fee exhibits, so an old rate is not used to judge a new date of service — or the reverse.
- The contract’s dispute, reconsideration, and corrected-claim procedures and deadlines, recorded alongside the rates.
- Any carve-outs, case rates, or codes paid under a different schedule.
Store this in the billing system if it supports contract modeling; otherwise a structured spreadsheet by payer, plan, code, and effective date works, provided one person owns updates and every change is dated. Verify the model before trusting it: run a recent set of remittances from each major payer, and investigate every large difference by hand. Some will be model errors — a missed amendment, a modifier rule entered wrong. Fix the model first, or staff will ignore a system that cries wolf, and a model wrong in the payer’s favor will hide the underpayments it was built to find.
Step 2: Compare expected vs. paid on every remit
Compare at payment posting, not month end. For each line, compare expected allowed and expected payment with what the Electronic Remittance Advice (ERA) or Explanation of Benefits (EOB) shows, after reading its Claim Adjustment Reason Codes and Remark Codes. Sort every variance into one of four explanations first:
- Patient responsibility explains it. The allowed amount is correct and the shortfall is deductible, copayment, or coinsurance correctly assigned to the patient. Bill the patient correctly instead.
- The practice’s own claim explains it. Units, a modifier, a code, or place of service differ from what was intended, and the payer paid the claim as submitted. The fix is usually a corrected claim, not a payment dispute.
- The model explains it. The contract data was outdated or entered incorrectly. Fix the model and re-run the affected dates, because other lines were judged against the same wrong rate.
- None of the above explains it. The allowed amount or payment is below the contract on a correctly submitted claim. This is a candidate underpayment, and it goes to the variance queue.
That sorting matters more than the software. Without it, staff dispute claims the patient owes and miss the pattern underneath the noise. Our underpayment recovery service page and guide to insurance underpayment recovery for practices describe the recovery work that follows; this queue feeds it.
Step 3: Set a variance threshold — and a pattern rule
No practice can dispute every one-dollar difference individually, and none should ignore small differences that repeat. Two rules work together.
The first is a single-claim threshold: the dollar amount, percentage, or combination below which an individual variance is logged and watched rather than disputed on its own. Set it from your own economics — the staff time a dispute takes, the payer’s process, and the size of the variance — and write it down as policy. There is no universal correct threshold.
The second is a pattern rule, and it is more important. Group sub-threshold variances by payer, plan, code, and reason, and review them on a set cadence. When the same shortfall repeats on the same code, the total is no longer small, and the cause is usually systematic: an outdated fee schedule on the payer’s side, a modifier rule applied differently than the contract states, or an unimplemented rate amendment. Dispute patterns as a pattern, with the full claim list attached. If pattern reviews keep finding large totals that sat below the threshold for months, tighten the cadence.
A worked example, with round numbers
The following is a hypothetical illustration with round numbers, created to show the arithmetic of the method. It is not a client case, not a Fastora result, and not a representation of any payer’s actual rates.
Suppose a contract sets the expected allowed amount for one frequently billed code at $100 per unit. Over one month, the practice bills that code on 80 claims. The remittances show an allowed amount of $92 on most of those lines instead of $100, with patient responsibility correctly shown as $0 on the illustrated lines.
The per-line variance is $8 — under a hypothetical $10 single-claim threshold, no individual claim would be disputed alone. The pattern rule catches it: 80 lines × $8 = $640 in one month, on one code from one payer; over a quarter, $1,920. File one pattern dispute citing the $100 contract rate vs. the $92 allowed, not 80 calls about $8. A variance too small to work alone becomes material, and provable, when it repeats.
Step 4: Assemble the dispute packet
Payers resolve documented disputes faster than described ones. The packet should let a reviewer verify the claim without asking for basics:
- The contract excerpt or fee exhibit page showing the rate that applies, with effective dates highlighted.
- A claim-by-claim list: claim control number, date of service, code and modifiers, units, expected allowed, actual allowed, patient responsibility applied, and variance.
- The remittance excerpt for each claim, showing the adjustment and remark codes the payer used.
- A short cover statement naming the pattern: payer and plan, code, date range, number of claims, and total variance.
- For pattern disputes, one or two fully worked sample claims showing the arithmetic line by line.
Submit through the channel the contract specifies, log the date, reference number, and response deadline, and diary follow-up before that deadline. Keep the packet with the contract file: a pattern recurring next quarter is a different conversation when the practice can show it was already identified and corrected once.
Step 5: Escalate patterns, and feed the findings back
Start with the payer’s standard dispute process. If it corrects the listed claims but the same variance appears on new remittances, reopen with the new list and prior reference number, and ask for the underlying cause — typically which fee schedule the payer has loaded — to be corrected, not only the payments. Follow any further contract steps in order and within their deadlines, and involve the practice’s own legal or contracting adviser before treating a disagreement as more than a billing dispute; this is an operational method, not legal advice.
Close the loop internally: update the model, tighten claim edits, shorten that payer’s review cadence, or carry the finding into the next contract negotiation with dates and totals. Report variances by payer and code, dollars disputed and resolved, days to resolution, and patterns still open — the practice’s own numbers, from its own remittances.
Frequently asked questions
How do I identify insurance underpayments in medical billing?
Build the expected amount from your contract for each code, then compare it line by line with the allowed amount and payment on every ERA or EOB, after separating patient responsibility. A shortfall none of those three can explain is a candidate underpayment.
What is the difference between an underpayment and a contractual adjustment?
A contractual adjustment is the agreed difference between the billed charge and the contracted allowed amount; it cannot be collected. An underpayment is a shortfall below the contracted amount itself. Compare payments with the contracted rate, never the billed charge.
How small an underpayment is worth disputing?
Set a written single-claim threshold from your own costs and payer mix. Below it, group variances by payer, plan, and code instead of discarding them: a few dollars repeated across many claims is a pattern dispute with a material total.
What should an underpayment dispute packet include?
The contract rate excerpt with effective dates, a claim-by-claim list (expected allowed, actual allowed, patient responsibility, variance), remittance excerpts with adjustment codes, and a short statement of the pattern and total. Add one or two fully worked sample claims for a pattern dispute.
What if the payer says the payment was correct?
Re-check the other three explanations: patient responsibility, your own claim as submitted, and your contract model. If all three are correct and the contract still supports a higher amount, use the next step the contract or provider manual provides, within its deadline, keeping prior reference numbers in the new packet.
Is an underpayment the same as a denial?
No. A denial pays nothing and starts an appeal or correction workflow. An underpayment is a paid claim that paid too little, so it hides: the claim closes as paid unless someone compares payment with the contract.
Conclusion
Underpayments persist because they are quiet and buried in claims marked paid. A practice that loads its contracts into a model, compares every remittance line against it, sorts variances before disputing them, and escalates patterns with a verifiable packet will find shortfalls spot checks never surface — with the documentation to act on them.
If you suspect payers are paying below contract, or no one compares expected against paid, Fastora’s free medical billing audit is a practical start. Fastora’s billing work is priced as a percentage of collections or a flat monthly fee, and underpayment review belongs on every remit, not occasionally.
