Official CARC description: “Patient cannot be identified as our insured.” Here’s what it actually means, why it fires, and how to get the claim paid.
The payer can't match the person on the claim to a member. Almost always a data hygiene issue — ID typos, name changes, or the wrong plan's card scanned at intake.
Liability group: CO = Contractual Obligation — the provider absorbs it — the amount cannot be billed to the patient. If the denial is wrong, the money is recovered by correcting or appealing the claim, not by balance-billing.
Also written: CO-31, CO31, “denial code 31”, “31 denial code” or “reason code 31” — same CARC, different ERA formatting.
Reason code 31 also appears under another group on the ERA. The number is the reason; the prefix decides who absorbs the money. This page covers CO 31 (Contractual Obligation). If your ERA shows a different prefix, use the matching page:
Re-scan the card; verify ID, name spelling, DOB against the 271 response
Correct and resubmit — this is not an appeal
No match anywhere: run a coverage-discovery check, then patient outreach
Tighten front-desk card capture (photo both sides, verify at every visit)
No appeal path — only correction. Protect timely filing by documenting the original submission while you chase the right identity/payer.
Fill in the denial facts — use placeholders, never real patient data (the letter keeps [PATIENT NAME]-style fields so you can merge real details privately). A payer-ready draft with an enclosure checklist comes back in ~20 seconds.
Your appeal letter draft will appear here.
The official CARC description for CO 31 is: “Patient cannot be identified as our insured.” In practice: The payer can't match the person on the claim to a member. Almost always a data hygiene issue — ID typos, name changes, or the wrong plan's card scanned at intake.
Denial code 31 under the CO group (Contractual Obligation) means: Patient cannot be identified as our insured. The number is the reason; the CO prefix decides who is liable — the provider absorbs it — the amount cannot be billed to the patient. If the denial is wrong, the money is recovered by correcting or appealing the claim, not by balance-billing.
Yes. CO 31, CO-31, CO31 and “denial code 31” are the same CARC — clearinghouses, payer portals and PM systems just print it differently. Only the group prefix (CO, PR, OA or PI) changes the meaning, because it changes who has to absorb the amount.
CO stands for Contractual Obligation — the provider absorbs it — the amount cannot be billed to the patient. If the denial is wrong, the money is recovered by correcting or appealing the claim, not by balance-billing.
No appeal path — only correction. Protect timely filing by documenting the original submission while you chase the right identity/payer.
The number 31 is the same reason code in both — what changes is the liability group. PR 31 is Patient Responsibility. CO 31 is Contractual Obligation, so the provider absorbs it. Check the prefix on the ERA before you write anything off or bill the patient — a mis-grouped 31 is one of the more winnable appeals.