CO 31 denial code

Claim adjustment reason code 31. Often reported as CO 31.

Payer cannot identify the patient

Unofficial reference. This is our own plain-English explanation, not the official code text. Payer rules differ. Verify with the payer and the official X12 code list. AlphaPrefix is not affiliated with X12, CMS or any payer. Data last updated Nov 11, 2026.

What 31 means

The payer could not match the patient on the claim to any of its members, so it could not process the claim.

The patient name, member ID or date of birth on the claim did not match a member in the payer's system. The payer is not saying the service is uncovered, only that it cannot find the person.

These are usually fixed by correcting the patient or subscriber data and resubmitting.

Group codes seen with 31

On a remittance, 31 comes after a group code. These are the ones it is most often reported with:

• CO 31 (Contractual Obligation): The provider's side of the ledger. The reduction comes from the payer contract or a rule the provider has to follow, and the patient normally cannot be billed for it.

The payer chooses the group code for each adjustment, so the same reason code can appear with another group code on your remittance. Read the two together. More about group codes.

Common causes

• The member ID was keyed incorrectly or an old ID card was used.
• The patient's name or date of birth does not match the plan's records, for example after a name change.
• A dependent was billed under the wrong subscriber or relationship code.
• The claim went to the wrong payer or the wrong plan within the payer.

What to do next

1. Copy the patient's current insurance card and compare every field with the claim.
2. Run an eligibility check to confirm the exact name, ID and date of birth the payer holds.
3. Send a corrected or new claim with the matching patient and subscriber details.
4. Ask the patient to update their information with the plan if the plan's record is wrong.

Related codes

• 32 (reason code) Patient not an eligible dependent
• 16 (reason code) Missing information or billing error
• 27 (reason code) Service after coverage ended