Denial code · group CO
CO-45: The difference between your charge and the payer's allowed amount under its fee schedule
Short answer
CO-45 is the contractual write-off on a priced line: the gap between what you billed and what the payer allows under your contract or its fee schedule. It is not a denial and needs no resubmission, only a check. Compare the allowed amount to the rate you loaded for that payer, and when the payer paid below contract, call or file a payment dispute. In network, the CO-45 amount is never billed to the patient.
Look up other codes in the denial code library.
Last reviewed by the Remitwright billing team. Editorial policy
Group CO means contractual obligation: the balance is written off, not billed to the patient. The number is the reason; the two-letter group says who carries the balance.
Common causes
The normal contract difference
Your charge master sits above the payer's allowed amount, as it should, and the payer writes the difference off as CO-45 on every paid line. This is how a priced claim looks when everything went right.
The wrong fee schedule applied
The payer priced the claim at an out-of-network rate, last year's rate, the wrong locality or the wrong product line. The CO-45 is larger than expected and the paid amount is smaller. New contracts that have not been loaded at the payer's end are the usual story in the first months after an effective date.
A pricing modifier or unit problem
A missing 26 or TC, a bilateral billed as two lines instead of one with modifier 50, or units above the payer's cap change the allowed amount. The payer reports the reduction under CO-45 rather than a more specific code.
A multiple procedure reduction
Second and later procedures on the same day are often paid at a reduced rate. Some payers show the cut as its own reason code; others fold it into CO-45, so the line looks like an ordinary write-off.
Out-of-network pricing
When you are not contracted, the payer's allowed amount comes from its own schedule, and the balance may be the patient's rather than yours. Whether you can bill it depends on your network status, the plan type and state and federal balance billing rules, not on the CO-45 label.
How to fix it and resubmit
- 1
Compare allowed to expected on every line
Load each payer's contracted rates into the practice management system and let posting compare the 835 allowed amount against them. A line that matches posts as a contractual adjustment with no further work. A line below contract goes to a variance queue.
- 2
Check the claim before you blame the payer
A missing pricing modifier, wrong units or a code the contract prices differently will produce a low allowed amount that is your error. Fix it with a corrected claim, frequency code 7, rather than a dispute.
- 3
Call with the contract page
For a short payment on a correct claim, call provider services with the claim number, the allowed amount you expected and the contract or fee schedule reference. Ask for a reprocessing and a reference number. A short payment is often a loading error at the payer and is fixed on the phone.
- 4
File a payment dispute when the call fails
Use the payer's reconsideration or payment dispute route, attach the contract page and the 835, and track the deadline. A dispute about pricing is not a medical necessity appeal and should not go in the appeals queue.
- 5
Post it right while you wait
The CO-45 amount posts as a contractual adjustment. Do not move it to the patient, and do not write off the variance until the dispute is closed. Keep the expected amount on the account so the balance stays visible on the aging.
How to prevent it
- Contracted fee schedules loaded per payer, per product and per effective date, with an underpayment variance report worked weekly
- A charge master reviewed each year so no charge sits below any payer's allowed amount, which would cap payment at the charge
- A new-contract checklist that confirms the payer has loaded the new rates by pricing the first claims against the schedule
- Pricing modifiers and unit rules in the scrubber so the allowed amount is reduced only when the payer's policy says so
A CO-45 that was really an underpayment
A cardiology group signs a new contract that takes effect in January. The February 835s show allowed amounts for its office visits and echocardiograms at the previous year's rates, so the CO-45 on every line is larger than the posting team expects. The variance report flags the lines. The biller calls the payer with the contract's rate exhibit and the claim numbers. The payer confirms the new schedule was never loaded, loads it, and reprocesses the claims in a later remittance. The posting team keeps the expected amounts on the accounts until the reprocessing posts. This is a hypothetical example.
Official text: the X12 claim adjustment reason code list. The meaning above is our paraphrase for billers; the list is the authority.
FAQ
Questions about CO-45
Is CO-45 a denial?
Can I bill the patient the CO-45 amount?
How do I know if the CO-45 is too big?
The payer applied an out-of-network rate but we are contracted. What now?
Should I lower my charges so CO-45 is smaller?
Next step
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While you wait: see the sample weekly report or read how to work claim denials. You can also download the sample weekly report (PDF).