Underpayment Detection Guide

Short answer

Underpayment detection compares the paid amount on each 835 ERA line against the expected amount from the payer contract and fee schedule. When the paid amount falls below the contracted allowed amount for reasons a contractual adjustment, co-payment, or deductible does not explain, the variance is a candidate for recovery. Industry estimates put underpayments and contract leakage at 3 to 5 percent of net revenue for typical practices, higher in specialties that bill complex multi-procedure encounters. CARC CO-45 ("charge exceeds fee schedule or maximum allowable") is the code most often misread as a required write-off when the real problem is a payer paying below the contracted rate. Recovery takes an accurate fee schedule loaded in the billing system, a tolerance-based routing workflow, and human judgment at every step. No automated system recovers underpayments on its own.

How big is the underpayment problem?

An underpayment looks like a payment, which is why it stays invisible. A payment that lands in the billing system as "paid" closes the claim in most practice management platforms. It is not a zero-balance denial; the payer paid something, so the claim drops off the follow-up queue even when the amount is wrong.

Industry analyses from MGMA and HFMA place payer underpayments at 3 to 5 percent of total contracted revenue. For a billing company managing a practice with $2 million in annual collections, that is $60,000 to $100,000 a year in revenue that was earned, billed, and apparently paid at the wrong amount. At $10 million in collections the same 3 percent loss is $300,000. Over five years it compounds to $1.5 million in a single practice.

MGMA's February 2025 poll of 193 practice leaders found 48 percent now audit payer payments monthly or quarterly, up from 26 percent who audited monthly in January 2023. One practice leader put it bluntly: "It's ridiculous how often [payer] payments are incorrect. Clinics that do not audit at least weekly are losing money." That is a practitioner assessment, not a vendor pitch, and it matches what contract auditing firms report. Systematic payer underpayment goes undetected for months or years because no individual claim triggers a review.

HFMA frames contract leakage as a distinct operational risk, separate from denial management, that deserves its own monitoring workflow. Commercial payers change fee schedules, apply carve-outs, and implement reduction policies, sometimes without notifying providers in advance. The gap between when a payer changes a rule and when a billing team catches it is what HFMA calls "policy drift," and the cost lands in the remittance data as a variance that looks like a clean payment.

How much of that leakage is recoverable depends on whether the contract is loaded accurately, whether the appeal window is still open, and whether the billing team can build a credible dispute from the contract language. No vendor can guarantee a recovery rate. What the data supports is that practices auditing payer payments at least monthly recover materially more than those that do not.

What counts as an underpayment vs a contractual adjustment

This distinction is where most underpayment leakage starts.

A contractual adjustment is the difference between the billed charge and the payer's allowed amount in the contract. If you bill $200 for a service and your contract with that payer allows $140, the $60 difference is a contractual adjustment. You post it as CO-45 (charge exceeds fee schedule or maximum allowable), write it off, and move on. You cannot bill the patient for that $60, and you cannot appeal it. It is the price of participation.

An underpayment is when the payer pays less than the contracted allowed amount: not less than your billed charge, but less than what the contract says the allowed amount should be. If your contract specifies $140 for that service and the payer sends $110, the $30 difference is not a contractual adjustment. The payer did not pay to the contracted rate. That is a recoverable underpayment.

Both situations arrive in the 835 ERA wearing the same CARC code, CO-45. The ERA reads "charge exceeds fee schedule or maximum allowable" either way. The code is correct because the payer applied a fee schedule, but it does not tell you whether the fee schedule the payer applied matches the one in your contract. That comparison is not in the 835. You have to load your contracted allowed amounts alongside the ERA data and run it yourself.

When a billing team posts CO-45 lines as contractual adjustments without checking against the contract, every payer underpayment in the batch becomes a permanent write-off. The revenue is gone at the moment of posting. This is the highest-risk failure mode in underpayment detection, and it stays invisible unless someone does the math.

The CMS Physician Fee Schedule is the public Medicare reference rate, published annually. For CY 2026, CMS finalized two conversion factors: $33.5675 for practitioners in qualifying alternative payment models and $33.4009 for all others. Commercial payer rates are negotiated separately and may be expressed as a percentage of Medicare (for example, 120 percent of Medicare for a given specialty) or as flat dollar amounts specific to the contract. Either way, the expected payment is deterministic once the contract terms are known, which is why loading the right fee schedule is the foundational step.

Where underpayments come from

A handful of mechanisms drive most of the underpayment volume in a typical billing company's book.

Payer error. Payers run high claim volumes through automated adjudication, and those systems apply incorrect fee schedules, map procedures to the wrong rate table, or apply an old rate after a contract renewal updated the terms. These are genuine processing errors and they are actionable. The appeal is simple: here is the contracted rate, here is what you paid, here is the difference. Many payers correct these quickly when the evidence is clean.

Fee schedule drift. Contracts renew on cycles, sometimes annually, sometimes every two or three years, and the rates in the billing system may not get updated when they do. A payer can also revise its internal fee schedule without notifying participating providers. Either way, the allowed amounts the billing team expects stop matching the amounts the payer applies. Drift cuts both directions: loaded rates that are too high turn correct payments into apparent underpayments, and loaded rates that are too low let real underpayments auto-approve. The MGMA four-step audit framework puts "collect and maintain current fee schedules" first because the detection logic is only as good as the reference data behind it.

Contract carve-outs. Most payer contracts reduce payment for specific services, categories, or billing scenarios. High-cost implants may carry a pass-through arrangement. Certain mental health services may have a separate rate table. Services at ambulatory surgery centers may pay differently than the same services in an office. Site-of-service differentials, outlier thresholds, and plan-specific rules are all carve-outs that have to be loaded into the expected-payment calculation. Skip them and the variance flags go wrong in both directions, flagging legitimate adjustments and missing real underpayments the carve-out logic never modeled.

Multiple procedure reductions. When a provider performs more than one procedure in a single encounter, payers apply multiple procedure payment reduction (MPPR) rules. Under Medicare policy the highest-valued procedure pays at 100 percent, and subsequent procedures at the same encounter are reduced by a percentage that varies by service category. For physical and occupational therapy services coded as "always therapy," the practice expense component is reduced by 50 percent for the second and later procedures. For certain diagnostic imaging procedures, the professional component of the second and subsequent procedures is reduced to a fraction of the Medicare fee schedule amount. Commercial payers often mirror Medicare MPPR policy, though the specifics vary by contract. A workflow that does not account for MPPR flags correct multi-procedure payments as underpayments and generates unwinnable appeals.

Capitation and withhold arrangements. In capitated contracts the payer pays a fixed per-member-per-month amount instead of a fee-for-service rate. Claims under a capitated arrangement should generate a zero-dollar ERA payment (or a token administrative amount) because the monthly cap covers the service, so treating that zero as an underpayment creates false alerts. Some contracts also include withhold provisions where the payer retains a percentage of payment pending performance or quality benchmarks and releases it quarterly or annually. Those withholds show up as reductions on individual ERAs but are not underpayments; they are deferred payments that should appear later in the PLB segment of a subsequent 835. A well-configured workflow routes capitated claims and withhold lines to separate handling rather than the recovery queue.

The detection workflow

Detection starts at ERA posting. Every 835 that arrives through the clearinghouse carries a paid amount for each service line plus the CARC and RARC codes that explain what the payer did. The logic compares two numbers: the paid amount from the 835 and the expected allowed amount derived from the payer contract and fee schedule.

The expected allowed amount is not a single lookup. It depends on the CPT or HCPCS code, any modifiers applied (modifier 25 for a separate E&M on the same day as a procedure, modifier 59 for distinct procedural services, modifier 50 for bilateral procedures), the units billed, the place of service, the provider's network status on the specific plan, and any contract carve-outs. For multi-procedure encounters, MPPR rules have to reduce the expected amount for the second and subsequent procedures before the comparison runs. Getting it right takes a rules engine that models the contract with enough fidelity to handle all of these inputs.

The resulting variance is then checked against a tolerance threshold. Tolerances exist for practical reasons: a $0.12 rounding difference on a $47 service is not worth human review, but a $180 shortfall on a $600 surgical procedure is. A common starting point flags any paid amount short of the allowed amount by $5 or more, with per-payer rules that raise the threshold for self-funded plans where appeal paths are longer and more expensive. A percentage threshold (flagging variances above 5 percent of the expected allowed amount) scales with dollar value automatically.

Variances above the threshold route to a review queue a billing specialist works like a denial follow-up queue. The routing should separate clear payer-error candidates (paid below the contract rate on a simple single-procedure claim with no carve-outs) from cases that need deeper investigation (a multi-procedure encounter where MPPR may apply, or a claim where a secondary payer already posted a payment that changes the net variance). Simple cases should surface with a pre-populated appeal letter. Complex cases should surface with full ERA context, the CARC and RARC pair, paid amount, calculated expected amount, and a link to the relevant contract section, so the specialist can make the call.

Timing is the other constraint. Most payer contracts impose appeal windows of 30 to 90 days from the date of payment, and both the 277CA acknowledgment date and the 835 receipt date are relevant reference points. A queue that does not surface items by days remaining to appeal is incomplete. The most important field in an underpayment workflow is the deadline, not the dollar amount.

See ERA posting in Medi for how expected payment context, actual remittance results, and CARC codes stay connected through the posting workflow.

Recovering underpayments: appeal, negotiate, or accept

Not every variance above the tolerance threshold is recoverable, and a billing team that appeals everything spends more on appeal labor than it recovers on a real fraction of items. The decision framework has three paths.

Appeal. Appeal when the payer error is clear and documentable: paid below the contracted rate on a straightforward claim, unambiguous contract language, an open appeal window, and a dollar amount that justifies the work. The letter cites the specific contract section, attaches the ERA, states the expected allowed amount and the paid amount, and requests the difference. Payers often correct genuine processing errors fast when the appeal is clean. Including the exact contract effective date and rate table reference heads off the most common payer response, which is to claim the rate you cited is no longer in effect.

Negotiate. Negotiate patterns rather than individual claims. When a payer systematically underpays a specific CPT code across a large volume, appealing each claim is labor-intensive. The better move is compiling the pattern (total claims affected, total variance by month, rate applied versus contracted rate) and bringing it to the payer as a contract compliance issue. Payers generally prefer to fix systemic errors through a settlement or a prospective rate correction over fielding individual appeals for months of claims. The conversation also gives you leverage at renewal, since documented underperformance against the contracted rate is a concrete data point.

Accept. Accept when a contract provision the billing team did not fully account for explains the variance, when the appeal window has expired, when the dollar amount is below the cost of appeal labor, or when investigation reveals a billing error on the practice's side. Accepting should be a deliberate decision with a documented reason, not the default outcome of never reviewing variances at all.

The denial management workflow guide covers appeal timelines and evidence-packet construction in depth. The underpayment appeal uses the same packet structure: ERA detail, claim detail, contract reference, and a clear statement of the discrepancy.

One path that gets underused is working underpayments as part of contract renegotiation. A billing company that tracks payer-specific underpayment rates over time accumulates evidence of systematic non-compliance. That evidence pays off at renewal: documented proof that a payer has consistently paid below contracted rates is a factual basis for demanding rate increases, retroactive corrections, or better terms. Treated as negotiating leverage rather than a recovery task, underpayment data can return more over time than individual appeals ever will.

Where does Medi fit?

Medi treats underpayment detection as a review and investigation workflow, not a revenue guarantee. It keeps expected payment context, actual remittance results, claim details, CARC and RARC codes, and recovery decisions connected, so the information a specialist needs to evaluate a suspected underpayment sits in one place instead of spread across the ERA file, the contract binder, and a spreadsheet.

The ERA posting workflow surfaces the paid amount alongside the claim's expected allowed amount when fee schedule data is loaded. Variances above tolerance route to a recovery queue. Each item carries the 835 context (paid amount, CARC, RARC, allowed amount) next to the claim detail and the deadline. Decisions are logged so future ERAs for the same payer and code pattern can reference prior outcomes.

Medi will not claim:

• That every underpayment will be detected without an accurate fee schedule loaded
• That every detected variance is recoverable
• That appeal letters generated by the system are ready to send without human review
• That recovery rates are predictable independent of payer, contract quality, and billing team execution

What Medi does claim is that underpayment work should be visible, filterable by payer and CPT and dollar amount, routed by deadline, and traceable from ERA receipt through final outcome. That is the workflow discipline that decides whether a billing company actually captures the revenue it has already earned.

See billing company operations for how underpayment tracking fits into the broader revenue cycle view across a multi-practice book of business.

What should buyers verify in an underpayment workflow?

Evaluating a vendor's underpayment detection takes specific questions. Claims about "AI-powered contract compliance" and "automatic underpayment recovery" are not useful. The questions below are.

• How does the system receive and store payer fee schedules? Is the loading process manual, or does the system maintain an active feed? How does it handle multiple fee schedules for the same payer across different plans and effective dates?
• What claim-level inputs does the expected-payment calculation use? Does it account for CPT code, modifiers, units, place of service, and provider network status, or just the CPT code alone?
• How does the system model multi-procedure MPPR rules? Can it distinguish a correctly reduced second-procedure payment from a payer error?
• What tolerance thresholds are configurable? Are thresholds set globally or by payer, plan, and financial class?
• How are variances routed? Does the routing logic distinguish clear payer-error candidates from items that need deeper investigation?
• How does the queue surface appeal deadlines? Is the deadline field a first-class sort and filter, or buried in a detail view?
• How are decisions logged? If a specialist accepts a variance as a legitimate carve-out, does that decision inform the handling of future similar claims from the same payer?
• Can variance data be aggregated by payer, CPT code, and time period for contract negotiation, or only for individual claim follow-up?
• How does the system handle capitated claims and withhold arrangements to prevent false underpayment flags?

The honest answer to most of these is that underpayment detection scales directly with the quality of the contract data loaded into the system. A vendor that glosses over this is promising outcomes it cannot deliver. A vendor that is direct about it is telling you where the real work is.

Frequently asked questions

What is an underpayment in medical billing?

An underpayment is when a payer remits less than the contracted allowed amount for a covered service. It is not a denial; the payer paid something. It is also not a contractual adjustment, the legitimate write-off of the difference between the billed charge and the contracted allowed amount. An underpayment is the gap between what the contract says the allowed amount should be and what the payer actually paid. That gap is recoverable when the contract terms are unambiguous, the appeal window is open, and the billing team has accurate fee schedule data loaded for the comparison.

How does CO-45 relate to underpayments?

CO-45 is the CARC code for "charge exceeds fee schedule or maximum allowable." It is correct for both a proper contractual adjustment and a payer underpayment, because in both the payer applied a fee schedule. The code does not tell you whether the fee schedule the payer applied matches the one in your contract. To answer that, compare the ERA's paid amount against the contracted allowed amount in your fee schedule data. When the paid amount equals the contracted allowed amount, CO-45 is a contractual write-off. When it falls below, CO-45 is concealing an underpayment. See the denial management workflow guide for the full CO-45 context.

What is the most common cause of underpayments going undetected?

Posting CO-45 lines as contractual adjustments at ERA posting without checking whether the paid amount matches the contracted rate. Most practice management and billing platforms auto-post CO-45 as a write-off. Unless the system compares the paid amount against a loaded fee schedule before approving the write-off, the underpayment becomes a permanent adjustment the moment it posts. Secondary causes: outdated fee schedules (the comparison runs against wrong data), failure to account for MPPR (legitimate multi-procedure reductions flagged as underpayments, breeding alert fatigue), and no systematic process for surfacing variances for review.

What appeal window do I have for underpayments?

Windows vary by payer and contract, but most commercial payer contracts impose deadlines of 30 to 90 days from the date of payment. Miss the deadline and you generally forfeit the recovery, since payers are not obligated to process late appeals. The 835 receipt date is the practical start of the clock for most purposes. A workflow that does not surface items by remaining appeal days is missing the most important triage dimension. Medicare's timely filing rules apply to claim submission, not underpayment appeals; underpayment disputes with Medicare follow the redetermination and appeal pathway, on different timelines.

Should I appeal every underpayment I find?

No. The call depends on the variance amount, the cost of appeal labor, the clarity of the contract evidence, whether the window is open, and the payer relationship. A $12 variance on a $95 service from a payer you have a strong relationship with may not be worth a formal appeal. A systematic $40 variance on a high-volume CPT code from a payer with a history of non-compliance is worth appealing individually and worth escalating to a contract-level conversation. The negotiation path, compiling the pattern and bringing it to the payer as a systemic compliance issue, often resolves faster and recovers more than claim-by-claim appeals.

How does multi-procedure reduction affect underpayment detection?

When multiple procedures are billed at the same encounter, Medicare and most commercial payers apply MPPR rules that reduce payment for the second and subsequent procedures below the full fee schedule rate. For certain therapy services, the practice expense component of additional procedures is reduced by 50 percent. For certain diagnostic imaging procedures, the professional component is reduced further. If the expected-payment calculation does not model MPPR correctly, the system flags correctly reduced multi-procedure payments as underpayments. That generates false alerts, wastes appeal labor on unwinnable disputes, and creates the alert fatigue that lets real underpayments slip through. Accurate MPPR modeling is not optional for any system used in specialties that bill multi-procedure encounters.

How does Medi distinguish an underpayment from a contractual adjustment?

Medi connects the 835 paid amount and CARC context to the expected allowed amount from the loaded fee schedule at ERA posting. When the paid amount falls below the expected allowed amount by more than the configured tolerance, the line routes to the underpayment review queue instead of auto-approving as a contractual write-off. The specialist sees the ERA detail (paid amount, CARC, RARC) alongside the expected allowed amount and the contract reference, then decides whether to appeal, accept, or investigate further. The decision is logged. Medi does not auto-approve underpayment write-offs without human review, and it does not auto-file appeals. Both are human decisions with system support.

How current is this guide?

Last reviewed 2026-06-07. Industry statistics draw from MGMA and HFMA research cited in the body. CMS Physician Fee Schedule references are from the CMS Physician Fee Schedule page and the CY 2026 final rule. CARC and RARC code definitions are maintained by X12. CMS remittance guidance is at Health Care Payment and Remittance Advice.