CO-15 Denial Code: Authorization Number Missing or Invalid

The official definition

“The authorization number is missing, invalid, or does not apply to the billed services or provider”

That is the verbatim definition of CARC 15 from the X12 Claim Adjustment Reason Code set, the standardized codes every insurer uses on EOBs and remittance advices. The letters in front of the number are the group code. CO: Contractual Obligation: an adjustment between the provider and the insurer. An in-network provider should not bill you for CO amounts.

What it means in plain English

The claim needed an authorization number and either didn't include one, or included one that's wrong: a typo, an expired authorization, or a number issued for a different provider or service. If authorization was actually obtained, this is one of the fastest denials to fix: add the right number, resubmit, done.

What to check on your EOB

• Whether authorization was actually obtained. Ask the provider's office for the authorization number and date.
• Whether the authorization matches the provider and service that were billed.
• The authorization's validity window against your date of service.

What to do next

1. If authorization exists, ask the provider's billing office to add or correct the number and resubmit. This is a simple data fix.
2. If authorization was never obtained, treat this as a CO-197 situation: the provider is responsible for the failure.

Who's responsible

Provider. Most denials carry a clear owner. Knowing whether the fix belongs to you, your doctor, or the billing office is half the battle. If it's the provider's error, you should not be paying for it.

Want the fundamentals first? Start with how to read an EOB and the 7 most common billing errors. This page is general information about standardized denial codes, not legal or medical advice.