Denial codes, explained by people who have coded
A denial code tells you a claim was adjusted. It rarely tells you what to do about it. These pages do: why you actually got each one, how to fix the one on your desk, and what would have stopped it before it left the building.
The prefix matters as much as the number. CO means the write-off is yours and you cannot bill the patient. PR means the money is the patient’s to pay and it is collectible. Reading a PR code as a denial is how practices give away revenue they were entitled to.
CO — Contractual Obligation
The write-off is yours. You agreed to it in your contract with the payer, and you cannot bill the patient for it.
- CO-16The claim is missing information the payer needs, or something on it is inconsistent.
- CO-97The service is considered already paid for as part of another service on the same claim.
- CO-45Your charge was above the payer's allowed amount, and the difference is a contractual write-off.
- CO-50The payer does not consider the service medically necessary for the diagnosis you submitted.
- CO-11The diagnosis does not match the procedure.
- CO-4The procedure code is inconsistent with the modifier used, or a required modifier is missing.
- CO-18Duplicate claim or service.
- CO-96Non-covered charge.
- CO-29The claim was filed after the payer's deadline.
- CO-109This claim is not payable by this payer or contractor.
- CO-151The payer says the information submitted does not support this many services.
PR — Patient Responsibility
The amount is the patient's to pay: deductible, coinsurance, or copay. These are not denials, and they are collectible.
Common questions
- What are denial codes?
- Denial codes are the standard codes a payer puts on a remittance advice to explain why a claim was reduced, adjusted, or not paid. They come in two kinds: claim adjustment reason codes, which say what the adjustment was, and remark codes, which add the detail. The group prefix matters as much as the number: CO means the write-off is yours contractually, while PR means the amount is the patient's to pay.
- What is the most common denial code?
- CO-16 is the most common. It means the claim is missing information or something on it is inconsistent, and on its own it tells you very little — the useful detail is in the remark codes on the same remittance line.
Most coding-side denials are visible before the claim leaves: paste the diagnosis codes into the free claim scrubber and it will flag the conflicts. The denial management page explains how the paid workspace runs those checks on every claim, and the ICD-10-CM lookup shows the coding notes behind any diagnosis code on the remittance.
Every explanation on these pages is our own. The official descriptions of the claim adjustment reason codes are published and copyrighted by X12, and we do not reproduce them — read the official list at X12. Reference information for professional coders and billers; not billing or legal advice.