CO-31 Denial Code: Patient Not Identified

The official definition

“Patient cannot be identified as our insured”

That is the verbatim definition of CARC 31 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 insurer couldn't match the claim to you. A transposed digit in the member ID, an outdated card on file, a name that doesn't match their records. Trivial cause, expensive consequence: claims denied this way often come back to you billed at full price as if you were uninsured, for a typo that was never yours.

What to check on your EOB

• The member ID and name on the claim against your current insurance card.
• Whether the provider has your newest card; this denial spikes after plan changes and re-enrollments.

What to do next

1. Give the provider's billing office your correct member information and ask them to resubmit.
2. Do not pay a full-price bill generated by this denial; it resolves with a corrected resubmission.
3. If the information was correct all along, call the insurer; the mismatch is on their side.

Who's responsible

Provider, with your correct card. 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.