CO-31 Denial Code: Patient Cannot Be Identified as Insured

CO - Contractual Obligation

Last updated: August 28, 2026

What CO-31 Means

The CO-31 denial code means the payer cannot identify the patient as its insured - the person on the claim does not match an active member in its records. It is a close cousin of remark code N382 (invalid patient identifier), but as a reason code it carries the denial itself: coverage could not be confirmed for this person on this date of service.

Common Causes

Coverage terminated before the date of service, so the patient is no longer on the payer's rolls
The member ID, name, or date of birth does not match the payer's enrollment records
The claim was billed to the wrong payer after a plan change the practice did not catch
A newborn or newly added dependent not yet loaded into the payer's system
The patient gave an old card at registration

How to Fix CO-31

Run a current eligibility check for the date of service to see whether coverage was active and under what ID
Correct any demographic or identifier mismatches and resubmit to the same payer if coverage was active
If coverage terminated, ask the patient for their active coverage and bill the correct payer - watch the new payer's timely filing limit
For newborns and new dependents, confirm enrollment was completed and resubmit once the member appears in the payer's system
If no coverage existed on the date of service, move the balance to patient responsibility per your financial policy

Frequently Asked Questions

What does denial code CO-31 mean?

CO-31 means the payer cannot identify the patient as its insured - the patient does not match an active member record for the date of service. It is usually an eligibility or identification problem: terminated coverage, a wrong ID, or the wrong payer.

How do I fix a CO-31 denial?

Verify eligibility for the date of service. If coverage was active, fix the ID or demographic mismatch and resubmit. If coverage had terminated, identify and bill the patient's actual payer, or move the balance to the patient if they were uninsured.

Related Denial Codes

N382

Missing or Invalid Patient Identifier

CO-22

Another Payer May Be Responsible (Coordination of Benefits)

CO-24

Covered by a Capitation Agreement or Managed Care Plan

Related Resources

Claim adjustment reason codes and remittance advice remark codes are standardized code sets maintained by X12. The explanations on this page are written in our own words for educational purposes, based on public CMS and Medicare contractor documentation. Payer policies vary - always confirm handling with the specific payer's guidance and your contracts.