Revenue cycle

Underpayments in medical billing: how to spot one on the remittance, and what to do about it

An underpayment is a claim the payer approved and paid, but for less than your contract or fee schedule says it owes. A denial at least shows up in a work queue. An underpayment gets posted, the balance is adjusted off, and nobody looks at it again unless someone compares what came in with what should have.

Underpayment, denial or contractual adjustment?

Three things reduce what you collect on a claim, and only one of them is an error on its face.

The second row is the problem: an underpayment and a correct contractual adjustment use the same codes. The only way to tell them apart is to know what the line should have paid.

Where underpayments hide on the remittance

Every adjustment on an 835 remittance carries a group code and a reason code. The group code says who owns the adjustment: CO (contractual obligation), PR (patient responsibility), OA (other adjustment) or PI (payor initiated reduction) (X12 group codes). These reason codes are where short payments most often sit:

• 45, charge exceeds the fee schedule or contracted amount. Correct when the allowed amount matches your contract; an underpayment when it doesn't. How to check a CO-45.
• 59, "processed based on multiple or concurrent procedure rules". Check which line took the reduction and by how much. Medicare, for example, pays the highest-valued surgery at 100% and the second through fifth at 50% (Claims Processing Manual ch. 12, §40.6). A reduction applied to the wrong line costs the difference. How the reductions work.
• 97, the service is included in another service's payment. Correct when the codes really bundle; an underpayment when a modifier should have kept them separate. How to test a CO-97.
• 253, sequestration. Traditional Medicare takes 2% off payment (CGS Medicare). Whether a Medicare Advantage plan may take it from you depends on your contract.

Each of these codes can be correct. What you're checking is whether the amount is.

Common causes

• The payer loaded the wrong rate. A contract amendment or annual update that never made it into the payer's system, or a rate tied to the wrong place of service.
• A percentage contract priced off the wrong base. Contracts written as a percentage of Medicare depend on the year's fee schedule and your locality. A plan using last year's rates or another locality underpays every line.
• Reductions applied wrong. Multiple-procedure reductions taken on the wrong line, or taken on services your contract exempts.
• Modifiers ignored. A modifier that should exempt a line from a bundling edit or a reduction is dropped in adjudication.
• Out-of-network Medicare Advantage paid below Medicare. A plan may not pay a non-contracted provider less than original Medicare would (42 CFR 422.214).

How to find them

1. Build the expected amount for every line. Use the contract terms, the fee schedule in effect on the date of service, and the reductions that legitimately apply. Here's how to calculate it.
2. Compare allowed, not paid. The paid amount drops patient responsibility (deductible, coinsurance, copay), which doesn't change what the payer owes overall. The allowed amount is what reflects the contract.
3. Look for patterns. One short line may be a one-off. The same code short by the same amount from the same payer is a loaded-rate problem, and fixing it at the source fixes every future claim.
4. Watch the clock from the remittance date. Appeal windows usually run from the date of the payment notice, not the date of service.

What to do when you find one

The Medicare Advantage payment dispute process covers amounts paid and down-coding. A claim paid at zero goes through the plan's appeal process instead (CMS memo).

When it's worth chasing

A single short line is often small, and working it costs staff time. Two things change the math. When the same error repeats across a payer's claims, one dispute about the loaded rate can recover the whole set and stop it happening again. And appeal windows close, so a backlog you meant to get to eventually becomes money you can no longer recover.

Questions

What is an underpayment in medical billing?

A claim the payer approved and paid, but for less than your contract or the applicable fee schedule says it owes. Nothing on the remittance calls it an underpayment; you find it by comparing the allowed amount on each line with what you expected.

Is an underpayment the same as a denial?

No. A denial pays nothing on the line and usually carries a reason you can act on. An underpayment pays something, so it is easy to post and move on.

How long do I have to dispute a Medicare underpayment?

For traditional Medicare, a redetermination must be filed within 120 calendar days of receiving the initial determination, which is presumed to be 5 days after the notice date (42 CFR 405.942). For commercial and contracted Medicare Advantage plans, the deadline is in your contract or the payer's provider manual.

Can a Medicare Advantage plan pay an out-of-network provider less than Medicare?

No. A non-contracted provider is paid what it could collect under original Medicare (42 CFR 422.214). If the plan pays less, you can dispute it with the plan and then with CMS's Payment Dispute Resolution Contractor.

Last reviewed October 19, 2026. This guide is general information about payer payment rules, not legal advice. Your payer contract and provider manual control the details; check them before you file.