CO-45 means your charge is higher than the payer's allowed amount under its fee schedule or your contract. The difference is a contractual write-off. On most claims that is routine. But CO-45 is also where underpayments hide, because a wrong allowed amount produces a CO-45 that looks exactly like a correct one.
1. What CO-45 means
- CO (Contractual Obligation): the provider writes off the difference and cannot bill the patient for it.
- 45: the charge exceeds the fee schedule, maximum allowable amount, or contracted or legislated fee arrangement.
Example: you bill $250. Your contract allows $140. The remittance shows $110 as CO-45, and the remaining $140 is split between the payer's payment and any patient responsibility (deductible, coinsurance, copay).
2. Why CO-45 is usually normal
Most practices set one charge master price above every payer's allowed amount, so nearly every paid line carries a CO-45. That is expected. It is why CO-45 is often excluded from denial reports: it is an adjustment, not a denial.
3. When CO-45 hides an underpayment
| Problem | What it looks like on the remittance |
|---|---|
| The payer loaded the wrong fee schedule | The allowed amount is lower than your contract rate, and the gap shows up as extra CO-45. |
| A rate change was not applied | Last year's rate is still being paid after a new contract year started. |
| A carve-out was paid at the base rate | A service with its own negotiated rate (for example imaging or a drug) was paid at a lower general rate. |
| A multiple procedure or site-of-service reduction was misapplied | The allowed amount is reduced in a way your contract does not allow. |
None of these produce a denial. The claim pays, the CO-45 absorbs the difference, and nobody looks again unless someone compares the allowed amount to the contract.
4. How to check CO-45 adjustments
- Load your contracted rates into your billing system or a spreadsheet, by payer and CPT code.
- Compare the allowed amount on every paid line to the expected rate. Flag any line where the allowed amount is lower.
- Group the variances by payer and code. One wrong rate repeats across every claim with that code, so the dollars add up fast.
- Dispute with the payer using the contract language and a list of affected claims. Check your contract for the dispute deadline.
Our contract underpayment recovery guide covers the full process.
5. Can you bill the patient for CO-45?
No. For in-network claims, the contract prohibits billing the patient for the difference between your charge and the allowed amount. That is balance billing. For Original Medicare, participating providers accept the Medicare-approved amount; non-participating providers who do not accept assignment can charge up to the limiting charge (115% of the non-participating fee schedule amount). Only the PR amounts on the remittance, like PR-1, are patient responsibility.
Related: CARC and RARC codes list, CO-96, and CO-97.
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