What does denial code CO-31 mean?

CO-31 means the payer could not identify the patient as one of its insured members. The claim never reached adjudication because no member record matched the identifiers submitted. Usually the coverage is real and one identifier is wrong.

Can I bill the patient for a CO-31 denial?

Only after confirming there was genuinely no coverage on the date of service, which an eligibility check will establish. If the coverage existed and the claim simply failed to match, correcting the identifiers is the provider's responsibility and the balance is not the patient's.

CO-31: The patient cannot be identified as an insured member

CO-31 means the claim never reached adjudication at all - the payer could not find the member. In most cases the coverage is real and one identifier is off by a character. The distinction that matters is between a matching failure, which is a correction, and genuine absence of coverage, which converts the balance to self-pay. Working these in bulk is efficient because the same registration or card-capture defect usually explains a cluster of them.

What it means: The payer's system found no member matching the identifiers on the claim, so it could not attach the claim to a policy.
Who pays the balance: Contractual while the cause is a data error; a patient with genuinely no coverage becomes self-pay.
Group code CO: Contractual Obligation - the provider absorbs the balance and cannot bill the patient

Why CO-31 happens

The member ID was entered without a required alpha prefix, or with a transposed digit.
The patient is a dependent and the claim was submitted under the dependent's ID rather than the subscriber's.
The name on the claim differs from the payer's record - a married name, a hyphenated surname, or a nickname.
Coverage had terminated before the date of service, so no active member record exists to match.
The patient presented a card from a different plan in the same family of payers, so the claim went to the wrong entity entirely.

How to fix a CO-31 denial

1
Run an eligibility check for the date of service
This separates the two causes immediately. A successful eligibility response means the coverage exists and one of your identifiers is wrong; a failure points to terminated or non-existent coverage.
2
Compare every identifier against the payer's response character by character
Member ID, subscriber relationship, name spelling, and date of birth all participate in the match. Fixing only the obvious one and resubmitting produces the same denial a second time.
3
Bill under the subscriber where the plan requires it
Many plans match dependents through the subscriber's ID with a dependent code. Submitting under the dependent's own number fails the match even when every character is correct.
4
Convert to self-pay only after confirming no coverage existed
Document the eligibility check before moving the balance to the patient. Reflexively converting a matching failure into a patient statement is how practices bill patients who were in fact covered.

Preventing CO-31 denials

Scan or photograph the insurance card at every visit rather than relying on a verbal update.
Run eligibility before the visit and resolve mismatches while the patient is still reachable.
Route the payer's returned member name back into registration so the spelling matches on the next claim.

CO-31 frequently asked questions

What does denial code CO-31 mean?: CO-31 means the payer could not identify the patient as one of its insured members. The claim never reached adjudication because no member record matched the identifiers submitted. Usually the coverage is real and one identifier is wrong.
Can I bill the patient for a CO-31 denial?: Only after confirming there was genuinely no coverage on the date of service, which an eligibility check will establish. If the coverage existed and the claim simply failed to match, correcting the identifiers is the provider's responsibility and the balance is not the patient's.

Related denial codes

CO-16 CO-27 CO-109 CO-140 PR-33

Last reviewed August 5, 2026.

Denial code explanations are original plain-English summaries written for reference and are not the official X12 code descriptions. Payer handling of any code varies by contract - always verify against the remittance advice and your payer agreement before adjusting a claim.