CO-31 means the payer could not find the patient in its member records with the details on the claim. Usually it is a wrong member ID, a name spelled differently, a wrong date of birth, or the wrong payer. Fix the demographics and resubmit.
The official wording
The X12 Claim Adjustment Reason Code list defines 31 as: "Patient cannot be identified as our insured."
The letters in front are the group code. CO means contractual obligation (the provider absorbs it), PR means patient responsibility, and OA is other adjustment. The same reason code can come with different group codes, so always read both.
Common causes
- Member ID typed wrong, or the prefix left off (common with BCBS plans).
- Name mismatch, such as a nickname, a hyphenated surname or a recent name change.
- Date of birth wrong by one digit.
- Dependent billed under the subscriber's details, or the reverse.
- The claim went to the wrong payer, for example original Medicare instead of a Medicare Advantage plan.
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How to fix it
- Run an eligibility check with the card in hand. Use the exact name and ID it returns.
- Fix the patient record, not just the claim, so the next claim goes out right.
- Resubmit. Watch the filing deadline with our timely filing calculator.
How to prevent it
- Scan the card at every visit and compare the ID with the record.
- Use real-time eligibility at check-in and copy the name exactly as the payer returns it.
One claim the payer could not match
| Field | On the claim | On the card | Result |
|---|---|---|---|
| Member ID | 123456789 | XYZ123456789 | Prefix left off |
| Name | Bob Smith | Robert Smith | Nickname used |
| Date of birth | 03/14/1980 | 03/14/1980 | OK |
| Remittance | CO-31 | No match |
Fixing the ID and the name in the patient record, then resubmitting, gets this paid. There is nothing to appeal.
Medicare and CO-31
Medicare can send this as PR-31. Common reasons are a Medicare Beneficiary Identifier (MBI) typed wrong, a name not entered exactly as on the card, or no Part B coverage on the date of service.
- Enter the MBI exactly as it appears on the Medicare card.
- Enter the first and last name in the same order and spelling as the card, including hyphens.
- If the card itself is wrong, the patient must call Social Security at 800-772-1213.
- If the patient has a Medicare Advantage plan, bill that plan instead.
CO-31 vs CO-27 vs CO-22
| Code | What it means |
|---|---|
| CO-31 | The payer cannot find this person |
| CO-27 | The payer found the person, but coverage had ended |
| CO-22 | The payer found the person, but another plan pays first |
How to fix CO-31
- Run eligibility with the card in hand.
- Copy the name, ID and date of birth exactly as the payer returns them.
- Check whether the patient is the subscriber or a dependent.
- Fix the patient record so future claims are right.
- Resubmit inside the filing limit.
CO-31 checklist
- ☐ The member ID includes any prefix.
- ☐ The name matches the card exactly.
- ☐ The date of birth is right.
- ☐ Subscriber and dependent details are not mixed up.
- ☐ The claim went to the right payer.
Questions people ask about CO-31
Can I bill the patient for CO-31?
No. The claim never matched a member. Fix the details and resubmit.
Is CO-31 the same as a terminated coverage denial?
No. CO-31 means the payer could not find the person. CO-27 means it found them but coverage had ended.
Is CO-31 the same as PR-31?
Same reason, different group code. Medicare often uses PR-31. Either way the fix is the same: correct the member details and resubmit.
What if the patient's Medicare card is wrong?
The patient must call the Social Security Administration at 800-772-1213 to correct it.
Do I need to appeal CO-31?
No. The claim never matched a member. Correct the details and send a new claim.
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All denial code guides
PR-1 · PR-2 · PR-3 · CO-4 · CO-5 · CO-11 · CO-15 · CO-16 · OA-18 · CO-22 · OA-23 · CO-24 · CO-26 · CO-27 · CO-29 · CO-31 · CO-45 · CO-49 · CO-50 · CO-59 · PR-96 · CO-97 · CO-109 · CO-119 · CO-129 · CO-140 · CO-146 · CO-151 · CO-167 · CO-170 · CO-181 · CO-185 · CO-197 · CO-198 · PR-204 · CO-234 · CO-236 · CO-242 · CO-252 · CO-253 · CO-288 · CO-A1 · CO-B7 · MA130 · N130 · N23 · N290
Timely filing limits by payer: Cigna timely filing limit · HealthSpring timely filing limit · Aetna timely filing limit · UnitedHealthcare timely filing limit · Humana timely filing limit · corrected claim timely filing limits
Sources: X12 Claim Adjustment Reason Code list (31), read October 27, 2026; CMS Medicare guidance where Medicare rules are mentioned; Noridian Medicare JE Part B reason code guidance, "Patient Cannot Be Identified" (PR-31), read October 27, 2026. Last reviewed October 27, 2026. General information, not coding or legal advice. Payer rules differ, so check the remark code and the payer's own policy.