Denial code CO-18

What does Denial code CO-18 mean on a medical bill?

CO-18 means the plan is looking at a claim it believes it has already handled: same patient, same date, same service. It stops the second copy instead of paying twice. Most of the time this is a billing office refiling a claim that was still in process, and the CO letters mark it as an adjustment between the plan and the provider under their contract rather than a balance for you. It starts to matter when the provider reads the stopped claim as unpaid and sends you the balance. It matters the other way too, because care that honestly happened twice on the same date needs a corrected claim rather than a dropped one.

What should I watch out for with Denial code CO-18?

• Ask the billing office whether the first claim was paid, and ask them for the EOB that goes with it
• Point out to the provider that a CO-18 line is a stopped duplicate, not a balance for you to cover
• Ask for a corrected claim when the service really did happen twice on the same date, with the times from your record