# CO 45 Denial Code: Contract Write Offs and Underpayments

> CO 45 means the charge exceeds the fee schedule or contracted rate. Learn when it is an expected write off and when it hides an underpayment.

Source: https://webillhealth.com/co-45-denial-code/  
Publisher: WeBill Health  
Last updated: November 4, 2026

Denial code

## CO 45 denial code: write off, or underpayment?

CO 45 is usually your agreed contract discount. It costs money when the payer allows less than the contract says.

[Request a Revenue Defense Audit](https://webillhealth.com/revenue-defense-audit/request/)

CO 45 often looks like a denial, but most of the time it is an expected adjustment. It records the difference between what you charged and what your contract or the fee schedule allows. The question that matters is whether the allowed amount matches the rate your payer agreed to.

* What CARC 45 says
Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement. [X12 reason codes](https://x12.org/codes/claim-adjustment-reason-codes)
* Group code CO
Contractual Obligation. [X12 group codes](https://x12.org/codes/claim-adjustment-group-codes)
* Who carries it
X12 allows CARC 45 only with group code PR or CO, depending on who is liable. [X12 reason codes](https://x12.org/codes/claim-adjustment-reason-codes)
* Billing the patient
Under Medicare assignment, the difference between the billed amount and the approved amount cannot be billed. [Noridian, Assignment](https://med.noridianmedicare.com/web/jfb/enrollment/nonparticipation/assignment-and-nonassignment-of-benefits)
* What Medicare assignment means
The provider agrees to accept the Medicare approved amount as full payment for covered services. [Noridian, Assignment](https://med.noridianmedicare.com/web/jfb/enrollment/nonparticipation/assignment-and-nonassignment-of-benefits)

## In the official words

> Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement

[X12](https://x12.org/codes/claim-adjustment-reason-codes)

> The difference between the billed amount and the Medicare approved amount cannot be billed.

[Noridian Healthcare Solutions](https://med.noridianmedicare.com/web/jfb/enrollment/nonparticipation/assignment-and-nonassignment-of-benefits)

## What CO 45 means for the patient bill

X12 allows CARC 45 only with group code PR or CO, depending upon liability, so read the group code on the line instead of assuming. [X12 reason codes](https://x12.org/codes/claim-adjustment-reason-codes) For Medicare claims under assignment, the provider agrees to accept the Medicare approved amount as full payment for covered services. The difference between the billed amount and the approved amount cannot be billed. The patient may still be billed coinsurance, any unmet deductible and services Medicare does not cover. [Noridian](https://med.noridianmedicare.com/web/jfb/enrollment/nonparticipation/assignment-and-nonassignment-of-benefits)

## Check the allowed amount against your contract

1. Load your contracted rate for each payer and each procedure code you bill.
2. Compare the allowed amount on every remit line with that rate.
3. Flag every line where the payer allowed less than the contract says, and group them by payer and code.
4. Dispute the variance with the payer under the terms of your agreement, and keep the remit and rate sheet with the dispute.

A CO 45 line that matches the contract is a normal write off. One that falls short of it is money the payer still owes you.

One check comes straight from X12: a CARC 45 adjustment amount cannot equal the total service or claim charge amount. A line where the whole charge was adjusted under CARC 45 deserves a second look. [X12 reason codes](https://x12.org/codes/claim-adjustment-reason-codes)

## Stop underpayments from hiding in CO 45

CARC 45 carries the gap between the charge and the fee arrangement, so an allowed amount that is too low posts as the same CO 45 line as a correct one. Only a comparison with the contract tells them apart.

The Revenue Defense Audit includes underpayments against your contracts as one of its checks.

## Sources

* [X12 reason codes](https://x12.org/codes/claim-adjustment-reason-codes)
* [X12 group codes](https://x12.org/codes/claim-adjustment-group-codes)
* [Noridian, Assignment](https://med.noridianmedicare.com/web/jfb/enrollment/nonparticipation/assignment-and-nonassignment-of-benefits)

## Denial Defense 2.0

Root cause denial work that closes the documentation gaps behind repeat denials before they disrupt your revenue cycle.

**98%**

clean claim rate

**40%**

A/R reduction within 90 days

**500+**

payer rulesets in our rule libraries

[See Denial Defense 2.0](https://webillhealth.com/denial-defense-2-0/)

## Related denial codes

These codes often appear beside CO 45 or clear the same way.

* [CO 16](https://webillhealth.com/co-16-denial-code/)
* [CO 22](https://webillhealth.com/co-22-denial-code/)
* [CO 97](https://webillhealth.com/co-97-denial-code/)

See every code on [Denial Codes Explained: What Each Means and How to Clear It](https://webillhealth.com/denial-codes/). When the same adjustment keeps returning, read the [Denial Root Cause Playbook](https://webillhealth.com/denial-root-cause-playbook/).

Reviewed by [Tyler Nguyen](https://webillhealth.com/tyler-nguyen/), Denial Defense Manager, on November 4, 2026. Facts verified November 1, 2026.

## Find out what your contracts still owe you

The Revenue Defense Audit is a free, forensic review of your claims and A/R by the team that would work your account. It flags payments that cleared below the rate your payer agreed to.

[Check my Revenue Defense Score](https://webillhealth.com/revenue-defense-score/?specialty=cardiology) for the denial exposure in your specialty.

[Request a Revenue Defense Audit](https://webillhealth.com/revenue-defense-audit/request/)

WeBill Health is a specialty focused revenue cycle management partner for independent practices. Request a Revenue Defense Audit at https://webillhealth.com/form/