Denial code CO-4

What does Denial code CO-4 mean on a medical bill?

CO-4 is a coding mismatch: the service code and the short flag attached to it, called a modifier, do not agree with each other, or a required flag never made it onto the claim. Modifiers tell the plan things like which side of the body was treated, or that two separate procedures really did happen. When they conflict, the plan stops the claim rather than guessing. None of this is about whether you needed the care. It is the claim form that needs work, and only the provider's coding staff can send it back through. The CO letters keep the stopped amount on the provider's side of their contract.

What should I watch out for with Denial code CO-4?

• Ask the billing office to review the modifier on that line and refile the corrected claim with your plan
• Ask whether the side of the body on the claim matches the side actually treated, when that applies
• Ask the provider to hold the balance while the corrected claim is in process rather than sending it to collections