Claims · Updated 2026-09-08 · 4 min read

Is CO-45 on a dental EOB a denial?

CO-45 is the in-network write-off: the gap between the office's fee and the plan's allowed fee. It rides paid lines and is never the patient's money.

CO-45 says the charge exceeds the fee schedule or contracted amount, and on a dental EOB that is the ordinary in-network write-off: the gap between the office's full fee and the fee the plan allows under the participation agreement. It is not a denial and it is not a reduction the office can argue with. It rides claim lines that were paid in full, alongside the payment, which is exactly why it gets misread — a code appearing beside a zero looks like a refusal, and this one usually appears beside a payment. As of September 2026, across 37 dental practices, the code appeared on 8,951 remittance lines printed as a bare 45 from 51 different payers, plus 2,900 more printed as CO45 or CO-45. The CO group letters are the whole answer to who pays: contractual obligation, write it off, never bill the patient.

What we measured

Single code tokens on dental remittance lines, all dates: what we saw across 37 dental practices, measured September 2026.

Two readings. First, 45 is one of the most-printed codes on dental remittances and one of only about a dozen among the forty most-printed tokens that is unambiguously an X12 code — 51 unrelated insurance companies use it the same way. Second, and separately: lines actually denied for exceeding a fee schedule were 527 denied lines, 0.8% of denials across 37 practices. The write-off is common; the denial that sounds like it is rare. Those are different events and only one of them is a refusal.

What the carriers' own documents say

The most telling document evidence for CO-45 is an absence. Delta Dental publishes a crosswalk of 938 internal policy codes onto their X12 reason and remark codes — every waiting period, exclusion, frequency cap, bundling rule and clinical judgment it prints — and CARC 45 does not appear in it at all. The write-off is not in the denial library because the carrier does not treat it as a reason for anything. (Delta Dental — Policy ID Mapping for CARC/RARC Health Care Policy Codes, pinned at Delta Dental of California network scope.)

MetLife states the same rule from the other end, as a limit on what the patient can be charged, in its Preferred Dentist Program Resource Manual — a network-scope provider manual covering PDP, PDP Plus and its federal products:

"Whenever a participant receives any services from a participating dentist, the plan allowance applies. When the plan allowance is not reimbursed in full or in part by the benefit plan, any difference is the responsibility of the participant up to the plan allowance. The participant is not responsible for any amount that exceeds the plan allowance."

That last sentence is CO-45 written as a patient-protection clause. The amount above the plan allowance is not anyone's to collect.

Why it depends on the contract

CO-45 exists only because a contract exists. Out of network, there is no contracted allowance and so nothing to write off — the difference between the office's fee and what the plan pays is a balance, and whether it can be billed turns on state law and the plan's own out-of-network provisions, not on this code. In network, the same difference is a write-off the dentist agreed to before the patient walked in.

That is why the fee amount on a CO-45 line is not evidence of anything about the patient's benefits, and why the group letters matter more than the number. Every other adjustment on a dental line can, on some contract somewhere, be the patient's. CO-45 is never the patient's money — but the group letters are part of the claim, so read them rather than the number alone.

What to do

• Post CO-45 as the contractual adjustment and move on. There is no appeal, no resubmission, and no conversation with the patient.
• Do not record it as a denial reason. A write-off logged as a denial puts a refusal on a line the plan paid in full, and every report built on that record is then wrong about the same line.
• If a line carries only 45 and nothing else, read the money before reading the code. A payment beside it means the claim was paid, not denied.
• Check the allowed amount against the fee schedule you have on file. A CO-45 much larger than expected usually means the fee schedule in the practice management system is stale, not that the claim went wrong.
• Never quote a patient the difference between your fee and the allowed amount as their share. On an in-network claim it is not theirs.
• Distinguish 45 from 131. A claim-specific negotiated discount is a one-off reduction, not the standing fee schedule, and it should not be used to re-derive your contracted fees.

Numbers last refreshed September 2026.

All articles · Kaylie