All denial codes - how to appeal each one
Every health insurance denial in the US is tagged with a short code. The most-searched ones - CO-50 (medical necessity), CO-11 (missing prior authorization), CO-55 (experimental treatment), and CO-97 (out of network) - together account for the majority of denied claims. Each guide below explains what the code means, why insurers issue it, the typical appeal win rate, and the citations and exhibits you need to win.
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The complete list
- CO-50
Medical Necessity Denial (CO-50)
Insurer says the treatment your doctor ordered is 'not medically necessary.' Most common denial in the US.
Read appeal guide - CO-11
Prior Authorization Denial (CO-11)
Service required pre-approval that wasn't obtained, was missing, or didn't match what was billed.
Read appeal guide - CO-55
Experimental Treatment Denial (CO-55)
Insurer labelled an FDA-approved or guideline-recommended treatment 'experimental' or 'investigational.'
Read appeal guide - CO-45
Exceeds Fee Schedule Denial (CO-45)
The provider billed more than your insurer's allowable fee. Often a contractual write-off, not your responsibility.
Read appeal guide - PR-27
Filing Deadline Denial (PR-27)
Claim was submitted after the timely filing window. Almost always a provider billing error you can fight.
Read appeal guide - CO-204
Service Not Covered Denial (CO-204)
Service marked as outside your benefit. Often a coding error or overridden by state-mandated benefits.
Read appeal guide - PR-50
Step Therapy Denial (PR-50)
Insurer requires you to 'fail first' on a cheaper drug before covering the one your doctor prescribed.
Read appeal guide - CO-16
Missing Information Denial (CO-16)
Claim is missing a code, signature, NPI, or auth number. The fastest denial type to fix.
Read appeal guide - CO-97
Out-of-Network Provider Denial (CO-97)
Provider isn't in your insurer's network. The No Surprises Act may cap what you actually owe.
Read appeal guide - CO-18
Duplicate Claim Denial (CO-18)
Insurer thinks the claim is an exact copy of one they already paid. Often a system mismatch.
Read appeal guide - CO-27
Coverage Terminated Denial (CO-27)
Insurer says coverage had ended on the service date. COBRA, grace periods, and ERISA can reverse this.
Read appeal guide - MHPAEA
Mental Health Parity Denial (MHPAEA)
Mental health or addiction benefit is more restricted than comparable medical care - likely a federal MHPAEA violation.
Read appeal guide - CO-242
Referral Required Denial (CO-242)
HMO denied a specialist visit because no PCP referral was on file. Retroactive referrals often fix this.
Read appeal guide - PR-1
Deductible Not Met Denial (PR-1)
Claim was applied to your annual deductible. Worth auditing the accumulation math for errors.
Read appeal guide - NSA
Balance Billing Denial (No Surprises Act)
Surprise out-of-network bill from an in-network facility or emergency. The No Surprises Act likely protects you.
Read appeal guide - CO-22
Covered by Another Payer Denial (CO-22)
A CO-22 denial means your insurer believes the care may be covered by another payer under coordination of benefits (COB) rules. In plain ter
Read appeal guide - CO-96
Non-Covered Charge Denial (CO-96)
A CO-96 denial means the insurer is calling the charge 'non-covered,' but CO-96 by itself is deliberately uninformative. It is a generic cat
Read appeal guide - CO-167
Diagnosis Not Covered Denial (CO-167)
A CO-167 denial means the insurer is refusing payment because the diagnosis code on the claim is not covered for the service billed. In prac
Read appeal guide - CO-119
Benefit Maximum Reached Denial (CO-119)
A CO-119 denial means the benefit maximum for a time period or occurrence has been reached, for example, a cap on the number of physical the
Read appeal guide - CO-109
Wrong Payer Denial (CO-109)
A CO-109 denial means the claim or service is not covered by this payer or contractor, and must be sent to the correct payer or jurisdiction
Read appeal guide - PR-49
Preventive Service Denial (PR-49)
A PR-49 denial classifies a service as routine, preventive, or screening, and bills it to you as non-covered or as patient responsibility. T
Read appeal guide - CO-4
Modifier Error Denial (CO-4)
A CO-4 denial means the procedure code is inconsistent with the modifier used, or a required modifier is missing. Modifiers are two-characte
Read appeal guide - CO-31
Patient Not Identified Denial (CO-31)
A CO-31 denial means the insurer cannot identify the patient as its insured. The claim's member ID, name, date of birth, or other identifyin
Read appeal guide
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