CO-31: Patient Cannot Be Identified as Our Insured
The payer could not match the patient on your claim to anyone on its member list. It is almost always a front-desk data problem, and it is usually quick to fix.
What CO-31 Means
The official wording for code 31 is "Patient cannot be identified as our insured." The payer looked for the patient using the details on your claim and found no match. With the CO group code, you cannot bill the patient for it.
It is almost always a data problem at the front desk, not a coverage decision. That is good news, because it is usually quick to fix.
The Usual Causes
| Cause | Example |
|---|---|
| Name does not match | Nickname, missing hyphen, or a married name the payer does not have yet |
| Wrong date of birth | Day and month swapped, or a typo |
| Wrong or old member ID | Old card, a missing prefix, or the subscriber's ID used for a dependent |
| Wrong Medicare number | An old number instead of the 11-character MBI |
| Newborn billed too early | Baby billed under the mother's ID before the baby was added to the plan |
| Wrong payer or plan | The patient switched plans, or has Medicare Advantage instead of original Medicare |
| No coverage that day | Coverage had not started or had ended on the date of service |
How to Fix a CO-31 Denial
- Run an eligibility check for the exact date of service, through the payer portal or your clearinghouse.
- Compare every field. Check the name, date of birth, member ID, group number and the patient's relationship to the subscriber against the eligibility response.
- Correct the claim and resubmit it. If the payer processed and denied the claim, send a corrected claim with frequency code 7, unless the payer asks for a new claim. Our claim frequency code tool shows where the code goes.
- If the patient is not covered, find the right payer and bill them, or move the balance to self-pay under your policy.
The corrected claim must still reach the payer within its filing limit. Check your deadline with the timely filing calculator.
Medicare and CO-31
Since January 1, 2020, Medicare claims must use the Medicare Beneficiary Identifier (MBI), an 11-character number on the red, white and blue card. A claim with an old number, or a name and birth date that do not match Medicare's records, will not match. You can look up a patient's MBI in your Medicare contractor's portal.
If the patient has a Medicare Advantage plan, the claim belongs with that plan, not the Medicare contractor. See our Medicare billing guide.
CO-31 vs Similar Denials
| Code | What it means |
|---|---|
| CO-31 | The payer cannot find the patient at all |
| CO-27 | The patient was found, but coverage had ended |
| CO-109 | The claim belongs to another payer or contractor |
| B11 | The claim was sent to the wrong payer and should go elsewhere |
| CO-16 | Information is missing from the claim |
How to Stop CO-31 Before It Happens
- Scan the front and back of the insurance card at every visit, not just the first one.
- Run eligibility one or two days before the appointment, and again for new patients at check-in.
- Enter the name exactly as it appears on the card, not as the patient says it.
- Ask every patient, "Has your insurance changed?" at each visit.
- For newborns, confirm the date the baby was added to the plan before you bill.
Frequently Asked Questions
Reviewed by Hassan Raza Awan, Founder — 4+ years of hands-on U.S. medical billing experience. General billing information — verify against current CMS guidance and your payer contracts.
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