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CO-31 Denial Code: Patient Not Identified as Insured

CO-31 means the payer could not identify the patient as one of its insured members using the information on the claim. The member ID, name, or date of birth doesn't match, or the claim went to the wrong payer. Correct the identifiers and resubmit.

Quick facts

Code
CO-31 (CARC 31)
Status
Active In use since January 1, 1995; last modified September 30, 2007.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The common group. The provider must correct the identifying information or find the right payer; the patient isn't billed on this denial.
  • PR (Patient Responsibility): Sometimes used when the payer has no record of coverage and treats the patient as uninsured. Confirm there's truly no coverage before billing the patient.
Official description
Patient cannot be identified as our insured.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-31 means

CARC 31 says the patient cannot be identified as our insured. The payer took the member ID in box 1a, the patient’s name in box 2, and the date of birth in box 3, and found no matching member. It isn’t saying the service isn’t covered; it’s saying it doesn’t know who the patient is.

CO-31 is different from coverage-date problems like PR-26 or PR-27, where the payer knows the patient but they weren’t covered on that date. With CO-31, the payer can’t find a match at all, so the fix almost always starts with identifiers.

Common causes

  • Mistyped member ID or a missing prefix or suffix.
  • Subscriber’s ID used for a dependent when the plan issues each member a unique ID, or the reverse.
  • Outdated card after a plan change, employer change, or ID reissue. For Medicare, the MBI may have been reissued.
  • Name mismatch, such as a maiden name, hyphenated name, nickname, or suffix that doesn’t match enrollment.
  • Date of birth mismatch between the claim and the payer’s records.
  • Wrong payer. The patient has a Medicare Advantage or Medicaid managed care plan, and the claim went to traditional Medicare or fee-for-service Medicaid, or to the wrong commercial plan or network leasing company.
  • Wrong payer ID at the clearinghouse, sending the claim to a similarly named payer.

How to fix it

  1. Run an eligibility check for the date of service using the card information.
  2. Compare the claim to the payer’s record. Match member ID, name spelling, and DOB exactly.
  3. Check the payer ID and plan. Confirm the claim went to the right payer, including the correct Medicare Advantage or managed Medicaid plan.
  4. Correct the claim and resubmit. If the claim was rejected before adjudication, submit a new claim; if it was adjudicated, follow the payer’s instructions, often a corrected claim with resubmission code 7 in box 22.
  5. If the payer has no record, contact the patient for current coverage or a new card.
  6. Bill the patient only after confirming there was no coverage.

How to prevent it

  • Scan the card at every visit and compare it with the eligibility response.
  • Use the payer’s spelling of the name, exactly as returned in the eligibility response.
  • Verify Medicare Advantage and managed Medicaid enrollment before billing traditional programs.
  • Maintain payer ID mapping in your billing system and review it when payers merge or change clearinghouse IDs.
  • Validate identifiers before submission. A Claims Validator can flag member ID formats that don’t fit the payer.

See eligibility and COB denials for how identification problems fit into broader eligibility failures.

Specialty notes

Labs, imaging centers, and anesthesia groups rarely see the patient’s card directly and depend on registration data from another provider. Getting a copy of the card and eligibility response from the referring practice or hospital prevents many CO-31 denials.

Remark codes that may appear with CO-31

  • N382 (Missing/incomplete/invalid patient identifier.): The patient identifier is missing, incomplete, or invalid.
  • MA27 (Missing/incomplete/invalid entitlement number or name shown on the claim.): The entitlement number or name on the claim is missing or invalid, a Medicare signal to check the MBI and name.
  • MA61 (Missing/incomplete/invalid social security number.): Social Security number missing or invalid, for payers that still use it as an identifier.
  • N104 (This claim/service is not payable under our claims jurisdiction area.): Not payable under this contractor's jurisdiction; the claim belongs with a different Medicare contractor.
  • CO-140 (Patient/Insured health identification number and name do not match.): The member ID and name don't match each other, a narrower mismatch.
  • CO-32 (Our records indicate the patient is not an eligible dependent.): The patient is found but isn't an eligible dependent on the plan.
  • PR-26 / PR-27 (Service before coverage began or after it terminated.): The patient is known to the payer but wasn't covered on the date of service.
  • CO-109 (Claim/service not covered by this payer/contractor.): The claim belongs with another payer or contractor.
  • CO-16 (Claim/service lacks information or has submission/billing error(s).): Missing or invalid information, sometimes used with a patient ID remark instead of CO-31.

CO-31 FAQ

What is the most common cause of CO-31?

A member ID that is mistyped, outdated, or belongs to the subscriber instead of the dependent, or a name spelled differently from the payer's records. Plan changes and new ID cards are frequent triggers.

How do I fix CO-31?

Run an eligibility check using the patient's card, confirm the exact member ID, name, and DOB the payer has on file, correct the claim, and resubmit. If the payer has no record, ask the patient for current coverage.

Can I bill the patient for CO-31?

Not right away. Confirm whether the patient has coverage with this or another payer. If you verify they had no coverage on the date of service, bill according to your self-pay policy.

Why did Medicare deny with CO-31 when the patient has Medicare?

The claim may carry an old or reissued Medicare Beneficiary Identifier, a name that doesn't match the Medicare card exactly, or the patient may be in a Medicare Advantage plan that should be billed instead.