CO-31 denial code
Patient cannot be identified as our insured
How to fix it
Re-verify the member ID, name spelling and date of birth exactly as they appear on the card, then resubmit.
How to prevent it
Scan or photograph insurance cards at registration and verify electronically rather than transcribing by hand.
In practice
A claim returns CO-31 — the patient cannot be identified as our insured. The member ID was transcribed by hand from a photograph of an insurance card and one character was read incorrectly.
Nothing about the coverage was wrong. The patient is insured, the plan is active, and the service is covered. The claim simply did not match a record on the payer's system.
Re-verifying the member ID, name spelling and date of birth exactly as the payer holds them, then resubmitting, resolves it. The systematic fix is electronic eligibility verification, which returns the payer's own spelling rather than asking a person to read a card correctly.
What sits behind it
CO-31 is a matching failure. The payer is not saying the patient has no coverage — it is saying it cannot find them using the identifiers provided.
The most common causes are mechanical: a transposed digit in the member ID, a name entered as it is spoken rather than as the plan holds it, a hyphenated surname entered in one form on the card and another in the payer's system, a date of birth off by a digit, or a dependent billed under their own name where the plan is held in the subscriber's.
Manual transcription is the root cause of most of it, and electronic verification is the fix because it eliminates the transcription step entirely — the payer returns its own record, which is by definition the spelling that will match at adjudication.
Related codes
Terms used here — Eligibility Verification · Claim Scrubbing · Rejection
How we handle it — Eligibility Verification · Claims Management · Denial Management
Primary sources
The rules behind CO-31, at the bodies that publish them.
- Medicare Secondary Payer Manual (opens in a new tab)
Centers for Medicare & Medicaid Services — When Medicare pays second, and to whom the claim goes first. Coordination-of-benefits denials are resolved here rather than with the patient.
- Medicare Coverage Database (LCD/NCD) (opens in a new tab)
Centers for Medicare & Medicaid Services — Searchable national and local coverage determinations. The direct answer to whether a diagnosis supports medical necessity for a given procedure.
- Claim Adjustment Reason Codes (CARC) (opens in a new tab)
X12 — The authoritative, maintained CARC list. Our denial code lookup explains these in plain English; X12 is where the canonical definitions live.
Every denial code with a guide
Authorization
Bundling
Contractual
Coverage
Data quality
Documentation
Eligibility
Liability and workers comp
Patient responsibility
Provider eligibility
Timely filing
Looking for a different code? Search all 190 CARC and RARC codes
Questions about CO-31
No. It means the payer cannot match the patient to a policy using the identifiers submitted. The coverage is often perfectly active — the claim simply did not match a record, usually because of a transcription error in the member ID, name or date of birth.
Manual transcription. A transposed digit in the member ID, a name entered as spoken rather than as the plan holds it, a hyphenated surname recorded differently, or a dependent billed under their own name where the policy is in the subscriber's.
Re-verify the member ID, name spelling and date of birth exactly as they appear in the payer's record — not as they appear on the card, which may itself differ — then file a corrected claim.
Use electronic eligibility verification rather than manual card transcription. It returns the payer's own spelling, which is by definition the version that will match at adjudication.
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