Understanding Common Denial Codes (CO-16, CO-45, CO-97)
Every remittance explains adjustments with codes. Knowing how to read them turns a confusing statement into a clear to-do list. This guide explains the structure and the codes you will see most often.
Group codes and reason codes
A remittance pairs a group code with a reason code. Group codes include CO (contractual obligation, the amount the provider cannot bill the patient), PR (patient responsibility), OA (other adjustment) and PI (payer initiated reduction). The reason code then explains why.
Codes you will see often
- CO-16: claim lacks information needed to process it
- CO-4: procedure code inconsistent with the modifier
- CO-29: timely filing limit expired
- CO-45: charge exceeds the contracted or allowed amount
- CO-50: not deemed medically necessary
- CO-97: payment included in another service
- CO-197: authorization missing
- PR-1, PR-2, PR-3: deductible, coinsurance and copay
How to respond
Match the response to the reason. Missing information means correct and resubmit. Modifier and bundling issues need coding review and often an appeal with documentation. Timely filing needs proof of the original submission. Authorization denials need retro-review where allowed.
Know your deadlines
Timely filing and appeal windows differ by payer. Medicare allows a year from the date of service for filing, while many commercial payers allow shorter periods, so check each contract and record deadlines.
Build a denial log
Record each denial with payer, code, amount and outcome and review it monthly. Repeated codes show where processes need fixing, such as registration for CO-16 or scheduling for CO-197.
Example: reading a remittance line
A line shows group code CO with reason 97. This tells you the payer considers the service bundled into another paid service and the patient cannot be billed. Your next step is to check modifiers and documentation and appeal if the services were separate.
Frequently asked questions
Can I bill the patient for a CO denial?
Generally not, because CO means a contractual obligation. Check your payer contract.
Where do I find the codes?
They appear on the remittance advice (ERA or EOB) next to each adjusted line.
Related services
- Medical Coding (CPT, ICD-10)
- Prior Authorization
- Denial Management & A/R Recovery
- Revenue Integrity Audit
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