Common Claim Denial Codes, Explained for Billing Teams
A denial code tells you why a payer did not pay a claim as billed. It does not tell you what to do next. This reference covers the denials billing teams see most often, with the usual root causes, a step-by-step workflow for each, and how to stop it recurring.
Each entry is written from the working side of the revenue cycle: what the denial really means, whether to correct, appeal, or adjust, and who on your team should own the fix.
Descriptions are plain-English summaries. The official CARC and RARC lists are maintained by X12.
A denial on an 835 or EOB has three parts: a group code (CO, PR, OA, or PI) that says who is financially responsible, a Claim Adjustment Reason Code (CARC) that says why the payment was adjusted, and often one or more Remittance Advice Remark Codes (RARCs) that add detail. CO-197, for example, means the provider is responsible (CO) because authorization was missing (CARC 197). Start with the CARC to understand the reason, read the RARC for specifics, and use the group code to decide whether any balance can go to the patient.
Eligibility & coverage denials.
CO-22: Care May Be Covered by Another Payer
CO-22 (often OA-22) means the payer believes another plan is primary under coordination of benefits. How to confirm coverage order and get the claim paid.
Read about CO-22 →CO-27: Expenses Incurred After Coverage Terminated
CO-27 (often PR-27) means coverage was not active on the date of service. How to verify eligibility, find new coverage, and bill correctly before timely filing.
Read about CO-27 →CO-31: Patient Cannot Be Identified as Insured
CO-31 means the payer cannot match the patient to a member on its files. How to fix member ID, name, and date of birth mismatches and resubmit the claim.
Read about CO-31 →CO-96: Non-Covered Charge
CO-96 means the payer considers the service a non-covered charge. How to read the RARC, verify benefits, and decide whether to appeal or bill the patient.
Read about CO-96 →CO-109: Claim Not Covered by This Payer
CO-109 means the claim was sent to the wrong payer or contractor. How to identify the correct payer, including carve-outs and Medicare Advantage, and rebill.
Read about CO-109 →CO-204: Service Not Covered Under Current Benefit Plan
CO-204 (often PR-204) means the service is not covered under the patient's current benefit plan. How to verify benefits and handle patient responsibility.
Read about CO-204 →Coding denials.
CO-4: Procedure Code Inconsistent With Modifier
CO-4 means the modifier on the claim is missing, invalid, or does not fit the procedure code. Common causes, how to correct the claim, and how to prevent it.
Read about CO-4 →CO-11: Diagnosis Inconsistent With Procedure
CO-11 means the diagnosis on the claim does not support the procedure billed. What triggers it, how to review pointers and documentation, and how to fix it.
Read about CO-11 →CO-97: Service Bundled Into Another Service
CO-97 means the service was bundled into another procedure already paid. How to check NCCI edits and global periods, and when a modifier is appropriate.
Read about CO-97 →CO-236: Procedure or Modifier Combination Not Compatible
CO-236 means two procedures or modifiers billed on the same day are incompatible under NCCI or state rules. How to check the edit and correct the claim.
Read about CO-236 →Medical necessity denials.
CO-50: Not Deemed Medically Necessary
CO-50 means the payer decided the service was not medically necessary. How to review policy and documentation, build an appeal, and handle ABNs for Medicare.
Read about CO-50 →CO-151: Information Does Not Support Frequency or Units
CO-151 means the payer says the information submitted does not support this many services or units. How to check unit limits, documentation, and appeal.
Read about CO-151 →CO-167: Diagnosis Not Covered
CO-167 means the diagnosis on the claim is not covered for the service billed. How to check coverage policy, review documentation, and correct or appeal.
Read about CO-167 →Claim data & filing denials.
CO-16: Claim Lacks Information or Has Billing Errors
CO-16 means the claim is missing information or has a billing error. How to use the RARC to find what is missing, correct it, and stop it from recurring.
Read about CO-16 →CO-18: Exact Duplicate Claim or Service
CO-18 means the payer sees the claim or service as an exact duplicate. Why it happens, how to check the original claim, and when a corrected claim is needed.
Read about CO-18 →CO-29: Timely Filing Limit Expired
CO-29 means the claim was received after the payer's timely filing limit. What proof of timely filing works, when to appeal, and how to stop these write-offs.
Read about CO-29 →CO-170: Not Payable When Billed by This Provider Type
CO-170 means the payer will not pay this service when billed by this provider type. How enrollment, taxonomy, and credentialing gaps cause it and how to fix it.
Read about CO-170 →CO-252: Attachment or Documentation Required
CO-252 means the payer needs an attachment or other documentation to adjudicate the claim. How to send records correctly and avoid repeat requests.
Read about CO-252 →Who is responsible for the balance.
The same reason code can appear with different group codes. The group code decides where the balance goes.
Questions about denial codes.
Want to know what denials are costing you? Try the denial cost calculator →
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