CO-18 Denial — How to Appeal for Behavioral Health
CO-18 (duplicate claim) is one of the most common insurance denial codes behavioral health therapists encounter. This guide explains what triggered the denial and the most effective appeal strategy across all major payers.
What CO-18 means
The payer believes this claim has already been submitted and paid (or denied). This can happen when a clearinghouse resubmits, when two claims share the same date, CPT code, and provider, or when the payer's system flags a resubmission as a duplicate.
Appeal strategy for CO-18
Pull the original claim and the RA for any prior payment or denial. If the original was paid, provide the remittance showing payment to close the loop. If the original was denied and you are legitimately resubmitting, include the original claim number and the RA for the denial to demonstrate this is a corrected resubmission, not a duplicate.
CPT codes commonly denied with CO-18
Appeal CO-18 by payer
Select your payer for appeal address, deadline, and payer-specific MHPAEA notes.
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Generate my appeal letter →Frequently asked questions about CO-18
What does CO-18 mean on a behavioral health claim?
The payer believes this claim has already been submitted and paid (or denied). This can happen when a clearinghouse resubmits, when two claims share the same date, CPT code, and provider, or when the payer's system flags a resubmission as a duplicate.
How do I appeal a CO-18 denial?
Pull the original claim and the RA for any prior payment or denial. If the original was paid, provide the remittance showing payment to close the loop. If the original was denied and you are legitimately resubmitting, include the original claim number and the RA for the denial to demonstrate this is a corrected resubmission, not a duplicate.
Does MHPAEA apply to CO-18 denials?
CO-18 is a general adjudication code that typically applies equally to all claim types. MHPAEA may still be relevant if the payer applies the underlying rule differently to mental health services than to comparable medical/surgical services.
Which insurers most commonly issue a CO-18 denial?
All major commercial payers — Aetna, Cigna, Anthem, UnitedHealthcare, Humana, and BCBS plans — use CO-18. Government programs (Medicare, Medicaid, TRICARE) also issue this code. The appeal strategy is similar across payers, but deadlines and submission addresses vary. Select your payer from the list below for payer-specific instructions.