Denial Codes

CO-45 Denial Code: The Contractual Adjustment, and When It Is Wrong

Why CO-45 is usually a routine write-off rather than a denial, the fee-schedule math with worked examples, and the four situations where a CO-45 is costing you money.

CO-45 is the code a payer uses to write off the part of your charge above the amount it allows for the service, so on most claims it is a contractual adjustment, not a denial. X12 defines Claim Adjustment Reason Code 45 as “Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement.” The CO group, Contractual Obligation, means the provider absorbs the difference; the patient is not billed for it.

Most CO-45s should be posted and forgotten. The ones worth your time are the ones where the allowed amount is wrong, and you can only spot those if you know what the allowed amount should have been.

Last updated October 23, 2026. Reviewed by Jacob Greenberg, ClaimCarePro.

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The official CO-45 definition

Quoted from the X12 Claim Adjustment Reason Code list, read October 23, 2026.

“Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement. Usage: This adjustment amount cannot equal the total service or claim charge amount; and must not duplicate provider adjustment amounts (payments and contractual reductions) that have resulted from prior payer(s) adjudication. (Use only with Group Codes PR or CO depending upon liability)”
X12, Claim Adjustment Reason Codes, code 45 (start 01/01/1995, last modified 07/01/2017)

The usage note gives you two tests most billers never apply. A CO-45 can never equal the full charge, so a line where CO-45 wipes out everything is really a denial with the wrong code. And a secondary payer must not repeat the primary payer's contractual reduction; the impact of a prior payer belongs under CARC 23, which X12 says is used only with group code OA.

Contractual adjustment vs denial: what CO-45 actually is

CO-45 contractual adjustment compared with a denial
	CO-45 contractual adjustment	A denial (e.g. CO-16, CO-97)
Did the payer pay the line?	Usually yes, up to the allowed amount	No, or not that line
What the amount is	Billed charge minus allowed amount	All or most of the charge
What you do	Post as a write-off; check the allowed amount	Fix, resubmit or appeal
Patient owes	Only the PR amounts on the line	Nothing until the claim is resolved
When to act	Allowed amount below your contract	Every time

Our billing software classifies CO-45 the same way: the standard contractual write-off, no action, and the patient must not be billed for it. Denials such as CO-16 and CO-97 go to a work queue instead.

The fee-schedule math, with worked examples

Dollar figures are illustrative, not any payer's actual rates.

Every line on a remittance balances the same way: billed charge = payer payment + CO-45 + patient responsibility (PR). The CO-45 is billed charge minus the allowed amount.

Worked CO-45 examples
Line item	Commercial in-network	Medicare, participating	Charge below the fee schedule
Billed charge	$200.00	$150.00	$95.00
Allowed amount	$118.00 (contract rate)	$110.00 (fee schedule amount)	$95.00 (lower of charge or $110.00 fee schedule)
CO-45 adjustment	$82.00	$40.00	$0.00
Patient responsibility (PR)	$30.00 copay (PR-3)	$22.00 coinsurance (PR-2), deductible already met	$19.00 coinsurance (PR-2)
Payer payment	$88.00	$88.00 (80 percent of allowed)	$76.00 (80 percent of allowed)
Check: payment + CO-45 + PR	$200.00	$150.00	$95.00

The Medicare columns follow Medicare's own rule: the allowed charge is “the lower of the actual charge or the fee schedule amount,” and Medicare pays 80 percent of the allowed charge after the deductible is met (Medicare Claims Processing Manual, Ch. 12 §20). The third column is the lesson: when your charge is below the fee schedule, there is no CO-45, and the payer pays on your lower charge. A chargemaster that sits below a payer's allowed amounts leaves money behind on every claim and never shows up as a denial.

Non-participating Medicare providers work differently. Medicare pays a nonparticipant no more than 95 percent of the fee schedule amount (Ch. 12 §20.4.1), and on unassigned claims the provider may charge the patient no more than the limiting charge, which is 115 percent of the nonparticipant fee schedule amount (Ch. 1 §30.3.12.3). Using the illustrative $110.00 fee schedule amount: the nonparticipant amount is $104.50 and the limiting charge is $120.18.

Which group codes CO-45 rides under, and what that means for the patient

X12 allows CARC 45 with exactly two group codes, CO or PR, “depending upon liability.” CO-45 is the in-network case: the contract or a fee arrangement set by law caps what you may collect, so the excess is yours to write off. Medicare frames the same idea for assignment: a provider who accepts assignment agrees to accept the Medicare allowed amount as the full charge (Ch. 1 §30.3), and Medicare's remark code MA13 warns that billing the patient for amounts not reported under PR can bring penalties.

PR-45 means the payer is telling you the patient, not the provider, is liable for the amount over the allowed. You mostly see it on out-of-network claims. Whether you can actually collect it depends on the patient's plan and on state and federal balance-billing rules, so check before you send a statement.

When CO-45 is wrong: the four situations worth working

When a CO-45 is wrong
Situation	How it shows up	What to do
Wrong fee schedule	Allowed amount below your contract: last year’s rates, an amendment the payer never loaded, a facility rate on an office visit, or a provider not linked to the group contract.	Compare allowed to the contract for the date of service. Send the fee schedule page and ask for reprocessing; appeal in writing if refused.
CO-45 equals the whole charge	Payment and patient share are zero and the entire charge is CO-45.	X12 says CARC 45 cannot equal the total charge. Call for the real reason and a reprocess.
Secondary repeats the primary	The secondary payer takes its own CO-45 on top of the primary’s contractual reduction.	Prior-payer impact belongs under OA-23. Ask the secondary to reprocess on the primary’s allowed amount.
Out of network, no agreement	A CO-45 from a payer you never signed with.	Ask which agreement or legislated fee schedule it applied and get a call reference. Appeal if there is none.

Two remark codes help here. N381 is “Alert: Consult our contractual agreement for restrictions/billing/payment information related to these charges,” which tells you the payer is pointing at a contract; ask which one. N130, “Consult plan benefit documents/guidelines for information about restrictions for this service,” points at the patient's plan instead.

Underpayments like these rarely trigger a denial queue, so they sit unnoticed and quietly lower what each visit is worth. Catching them is part of keeping A/R current; see how to reduce days in accounts receivable.

The CO-45 workflow

Balance the line. Payment plus CO-45 plus PR must equal the billed charge. If it does not, there is another adjustment you have not read.
Compare the allowed amount to your expected rate. Load each contract's fee schedule with its effective dates so this is automatic.
Post correct CO-45s as write-offs. Move only the PR amounts to the patient.
Work the exceptions. Underpaid lines go to the payer with the contract page and a call reference, then to a written appeal.
Review charges yearly. Any code where your charge is at or below a payer's allowed amount needs a charge review.

This is how our denial management service treats CO-45: posted when right, worked when it is not. Related codes are on the denial codes hub.

How to prevent CO-45 problems

Keep a current copy of every payer fee schedule and amendment, with effective dates.
Confirm each new provider is linked to the group contract before billing under it.
Bill the right place of service so the payer applies the right rate.
Set charges above the highest allowed amount you are paid for each code.
Verify network status at intake so out-of-network patients know what they may owe.

Not sure your allowed amounts are right?

ClaimCarePro works denied and aged A/R on a 10% contingency of what is actually paid: no recovery, no fee. Practices that want ongoing billing pay 6% of collections with a $1,000 monthly minimum, month-to-month, with denial work included. Start with a free A/R audit or call (844) 772-8902.

CO-45 questions, answered

What does the CO-45 denial code mean?

CO-45 means you billed more than the payer allows for the service, and the difference is a contractual write-off. X12 defines CARC 45 as “Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement.” With the CO group, the provider absorbs that difference and the patient is not billed for it.

Is CO-45 a denial?

Usually not. On a paid claim, CO-45 is the normal adjustment between your charge and the allowed amount. It becomes a problem only when the allowed amount is lower than your contract says, when the CO-45 wipes out the whole charge, or when a payer you have no contract with applies it.

Can I bill the patient for a CO-45 adjustment?

No, not when it is reported as CO. The patient owes what the payer reports under PR, such as deductible (CARC 1), coinsurance (CARC 2) and copay (CARC 3). Medicare also requires a provider who accepts assignment to accept the Medicare allowed amount as the full charge.

What is the difference between CO-45 and PR-45?

Same reason, different payer of the difference. X12 says CARC 45 is used only with group code PR or CO, “depending upon liability.” CO-45 puts the excess on the provider under a contract or legislated fee. PR-45 tells you the payer considers the patient liable for it, which you mostly see out of network.

How do I calculate the CO-45 amount?

Billed charge minus allowed amount equals the CO-45 adjustment. The allowed amount is then split between the payer’s payment and the patient’s share. A line balances when billed charge equals payment plus CO-45 plus patient responsibility.

When is a CO-45 wrong?

When the allowed amount does not match the fee schedule you agreed to (old year, missing amendment, wrong location or provider), when the CO-45 equals the entire charge, which X12 says it cannot, when a secondary payer repeats the primary payer’s contractual write-off, or when you are out of network and have no agreement with the payer.

Should I appeal a CO-45?

Only when it is wrong. A correct CO-45 is posted as a write-off. If the allowed amount is below your contract rate, send the payer the fee schedule page and ask for reprocessing; if that fails, appeal in writing. Do not send a corrected claim just to change the charge.

Why does my CO-45 vary from claim to claim for the same code?

Because your charge is the same but the allowed amount is not. Allowed amounts differ by payer, by plan, by year, by place of service and, for Medicare, by locality. A CO-45 that suddenly grows for one payer is the clearest early sign that a fee schedule changed or was loaded wrong.

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Last updated October 23, 2026. Reviewed by Jacob Greenberg, ClaimCarePro.

Sources

X12, Claim Adjustment Reason Codes (codes 1, 2, 3, 23, 45), read October 23, 2026
X12, Remittance Advice Remark Codes (MA13, N130, N381), read October 23, 2026
X12, Claim Adjustment Group Codes, read October 23, 2026
CMS, Medicare Claims Processing Manual, Chapter 12, §20 and §20.4.1 (Rev. 13753, May 7, 2026)
CMS, Medicare Claims Processing Manual, Chapter 1, §30.3 and §30.3.12.3 (Rev. 13725, April 10, 2026)