Quick answer: CO 45 means the amount you billed was higher than the maximum the payer allows for that service under your contract or fee schedule. The difference is written off as a contractual adjustment — and in most cases, you cannot bill the patient for it.
Strictly speaking, CO 45 is not a "denial" at all. The claim was usually processed and paid — just at the payer's allowed rate, not at your billed rate. But when CO 45 adjustments start climbing, or start appearing where they should not, they become one of the quietest ways a practice loses revenue without anyone noticing.
The official description of CO 45 is: "Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement." Breaking that down:
| Line item | Amount |
|---|---|
| You bill for an office visit | $220 |
| Payer's contracted allowed amount | $145 |
| Paid by payer + patient (copay/deductible/coinsurance) | $145 |
| CO 45 adjustment (written off) | $75 |
A certain amount of CO 45 is completely normal. Almost every practice bills above contracted rates — charge masters are usually set higher than any single payer's fee schedule, so a contractual adjustment on nearly every claim is expected.
CO 45 becomes a problem in three situations:
No — not if you are in-network. The "CO" group code specifically means the provider is contractually obligated to absorb the difference. Billing a patient for a contractual adjustment violates most payer contracts and, depending on the payer and state, can create compliance exposure. The patient's responsibility is limited to their share of the allowed amount — copay, coinsurance and deductible — which appears under PR (Patient Responsibility) codes on the ERA.
| Code | Meaning | Who pays |
|---|---|---|
| CO 45 | Charge exceeds fee schedule / contracted rate | Provider write-off |
| PR 1 / 2 / 3 | Deductible, coinsurance, copay | Patient |
| CO 97 | Service bundled into another paid service | Check modifiers first |
| OA 23 | Prior payer adjudication (coordination of benefits) | Depends on COB |
Confusing these is one of the most common posting errors we see when we take over a practice's billing — patient balances get written off as contractual, and contractual amounts get billed to patients. Both are expensive mistakes.
A correct CO 45 — a genuine contractual adjustment — is not appealable, because it reflects your signed contract. An incorrect CO 45, where the payer applied the wrong fee schedule or misused the code, absolutely is. The skill is telling the two apart, which requires knowing your contracted rates.
The most common causes are a payer fee schedule update, a contract renewal that changed rates, claims processing under the wrong network tier, or a change in your own charge amounts. Trend the adjustments by payer and code to isolate which one it is.
It heavily affects your gross collection rate (collections vs billed charges), which is one reason gross collection rate is a poor performance metric. Your net collection rate — collections vs what you were contractually entitled to — is the number that matters, and correct CO 45 posting is what makes it accurate.
Seeing more adjustments than you can explain?
Reviewing exactly this — whether your write-offs match your contracts — is part of the free billing audit we run for practices. Our denial management team categorises every adjustment by root cause, and our AR management workflow disputes the ones that should never have been written off.
Reviewed against the standard Claim Adjustment Reason Code (CARC) definitions maintained by X12. Payer-specific handling can vary — always confirm against your own contract terms.
