Medical Billing Denial Codes: The 10 Most Common, Explained

By Dr. Zain Vally | Updated November 2026 | Denial Management

A denial code is the reason a payer gives on the remittance for not paying a claim line in full. It pairs a group code that says who owes the balance with a Claim Adjustment Reason Code (CARC) that says why. In Optum's 2024 Revenue Cycle Denials Index, payers initially denied 12% of hospital claims in 2023.

The ten codes below cover the categories that index ranks highest, led by registration and eligibility, missing claim data and authorization. They are grouped by category, not ranked by frequency.

10 Most Common Denial Codes in Medical Billing

CO-16: Claim lacks information or has a billing error; the remark code names what to fix.
CO-4: The procedure code does not match the modifier used.
CO-97: The service is bundled into payment for another service already adjudicated.
CO-197: Precertification, authorization or notification is absent.
CO-22: Another payer may be primary under coordination of benefits.
CO-26: The service date falls before the patient's coverage started.
CO-50: The payer does not deem the service a medical necessity.
CO-18 / OA-18: Exact duplicate claim or service; X12 assigns this code to group OA except where state workers' compensation rules require CO.
CO-29: The time limit for filing has expired.
CO-45: The charge exceeds the fee schedule or contracted rate, a contractual reduction more than a true denial.

The full searchable list of codes is below.

Every claim denial on an ERA carries two pieces of information that determine what you do next. The CARC (Claim Adjustment Reason Code) tells you why the payer denied or adjusted the claim. In front of it, the group code tells you who is financially liable: CO (Contractual Obligation) means the provider absorbs the adjustment, PR (Patient Responsibility) means the patient owes, OA (Other Adjustment) covers coordination of benefits and secondary payer logic, and PI (Payer Initiated) flags reductions the payer made on its own, such as pricing corrections.

The table below covers every denial code we have published a fix guide for, grouped by category and linked to the full resolution page. Need to search by code number or keyword? Use our Denial Code Lookup Tool to search all 106 codes in the database.

All Denial Codes: Sortable Reference Table

Click any column header to sort. Use the search box to filter by code number, description, or category. Every code with a fix guide links directly to the resolution page.

Code	Description	Liable	Category	Guide
CO-22	Coordination of benefits: another payer is primary	CO	Eligibility	Fix guide
CO-26	Expenses incurred prior to coverage: service date outside active window	CO	Eligibility	Fix guide
PR-26	Expenses incurred prior to coverage: patient responsible (coverage lapsed)	PR	Eligibility	Fix guide
CO-55	Procedure, treatment or drug deemed experimental or investigational by the payer	CO	Coverage	Fix guide
CO-131	Claim does not meet payer-specific criteria for the network/plan	CO	Eligibility	Fix guide
CO-185	Fatal edit: claim data does not match payer's member record	CO	Eligibility	Fix guide
CO-140	Patient/Insured health identification number and name do not match.	CO	Eligibility	Fix guide
CO-16	Claim lacks information or has a submission/billing error; the remark code names the field	CO	Information	Fix guide
CO-4	Procedure code inconsistent with modifier, or required modifier missing	CO	Coding	Fix guide
CO-97	Procedure bundled into another service on the same date	CO	Coding	Fix guide
CO-206	National Drug Code (NDC) not valid for the date of service	CO	Coding	Fix guide
CO-208	National Drug Code (NDC) units, quantity, or form do not match	CO	Coding	Fix guide
CO-209	National Drug Code (NDC) not eligible under the patient's benefit plan	CO	Coding	Fix guide
CO-197	Precertification, authorization, or notification absent for billed service	CO	Authorization	Fix guide
CO-49	Routine/preventive exam: non-covered charge per contract	CO/PR	Provider	Fix guide
CO-18	Duplicate claim or service: exact or near-match already processed	CO	Duplicate/Filing	Fix guide
CO-29	Timely filing limit exceeded: claim received after payer deadline	CO	Duplicate/Filing	Fix guide
CO-150	Information submitted does not support this level of service	CO	Medical Necessity	Fix guide
CO-50	Non-covered because not deemed a medical necessity by the payer	CO	Medical Necessity	Fix guide
CO-21	Injury or illness is the liability of the no-fault carrier	CO	Contractual	Fix guide
CO-45	Charges exceed fee schedule or maximum allowable amount	CO	Contractual	Fix guide

No denial codes match your search. Try a different keyword or search all 106 codes in the lookup tool.

Frequently Asked Questions

What is the difference between CO and PR on a denial code?

CO (Contractual Obligation) means the provider absorbs the adjustment as a write-off under the payer contract, while PR (Patient Responsibility) means the patient owes the balance. The same CARC number can appear with either group code, and the group code determines who you pursue for payment. CO adjustments cannot be billed to the patient.

How do I look up a denial code that is not listed on this page?

Our Denial Code Lookup Tool contains all 106 CARC codes in the current X12 835 standard. Enter the code number or a keyword from the remittance description to find the plain-English explanation and resolution steps.

When is a denial appealable versus requiring a corrected claim?

A denial is appealable when the payer made an incorrect determination, such as denying a medically necessary service or applying the wrong fee schedule. Use a corrected claim when the original submission contained an error: wrong modifier, missing information, incorrect payer ID, or a coding mismatch. Filing an appeal on a claim that needs correction wastes time; filing a corrected claim on a valid denial waives your appeal rights with some payers.

What are the most common denial codes in medical billing?

The ten most common, grouped by category, are CO-16 (missing information or billing error), CO-4 (modifier mismatch), CO-97 (bundled service), CO-197 (authorization absent), CO-22 (coordination of benefits), CO-26 (service before coverage), CO-50 (medical necessity), CO-18 (duplicate claim), CO-29 (timely filing) and CO-45 (fee schedule reduction).

Is there a complete list of denial codes?

Yes. X12 maintains the complete list of Claim Adjustment Reason Codes at x12.org, including retired codes and their stop dates. This page covers the codes physician practices actually see, each linked to a fix guide where we have one.

Denials costing your group more than 5% of net revenue?

We audit denial queues for multi-location practices and build the workflows that prevent the same codes from recurring. The analysis is free.

Request a Denial Audit