CO · Contractual ObligationCARC 31

Denial Code CO-31

Q: What does CO-31 mean?

CO-31 is a Contractual Obligation (CO) denial: "Patient cannot be identified as our insured." The payer can't match the patient to an active member on its file as billed — the member ID, name, or date of birth doesn't tie to a covered person, or the claim went to the wrong plan entirely.

Q: Is CO-31 the patient's responsibility or a provider write-off?

CO-31 carries the CO group code (Contractual Obligation): Provider/contractual responsibility — not billable to the patient while denied. A CO adjustment is the provider's responsibility, not the patient's: balance-billing the patient for a CO amount is a contract (and often compliance) violation. But CO does not automatically mean “write it off.” When the cause is fixable — missing information (CO-16), a modifier problem (CO-4), or NCCI bundling (CO-97) — you correct and resubmit, or appeal with documentation. You only truly write the amount off when it is a final contractual adjustment, such as the fee-schedule difference on CO-45.

Patient cannot be identified as our insured.

Source: X12 Claim Adjustment Reason Codes (CARC) & Group Codes. Maintained by the D3rx Clinical Billing Team.

What CO-31 means

The payer can't match the patient to an active member on its file as billed — the member ID, name, or date of birth doesn't tie to a covered person, or the claim went to the wrong plan entirely.

Why CO-31 happens

A wrong or transposed subscriber/member ID number
The claim was sent to the wrong payer or the wrong plan within a payer
The patient's name or date of birth doesn't match the payer's enrollment record
Coverage wasn't active on the date of service, or the patient isn't actually enrolled

What to do when you get CO-31

1Re-verify eligibility from the card or payer portal — member ID, name, DOB, and plan
2Correct any mismatched identifier and resubmit a corrected claim
3If the patient is not this payer's member, refile to the correct payer
4If the patient truly has no active coverage, the balance may shift to the patient after notice

Got this denial right now?

Fix & resubmit: see the CO-31 correction steps

See CO-31 correction steps Ask D3 about CO-31

CO-31 clears with a corrected claim, not an appeal. Ask D3 walks you through the exact fix — free, no signup.

CO group code: who absorbs the charge

Contractual Obligation — Provider/contractual responsibility — not billable to the patient while denied.

A CO adjustment is the provider's responsibility, not the patient's: balance-billing the patient for a CO amount is a contract (and often compliance) violation. But CO does not automatically mean “write it off.” When the cause is fixable — missing information (CO-16), a modifier problem (CO-4), or NCCI bundling (CO-97) — you correct and resubmit, or appeal with documentation. You only truly write the amount off when it is a final contractual adjustment, such as the fee-schedule difference on CO-45.

Appeal, correct, or write off CO-31?

CO-31 is almost always a corrected claim or a refile to the right payer — not an appeal. Fix the member identifiers from verified eligibility and resubmit, or send the claim to the payer that actually covers the patient. Appeal only if you can document that the patient WAS this payer's active insured on the date of service and the data submitted was correct.

Timing & deadlines

Resubmit or refile within the payer's timely-filing limit (Medicare 12 months from date of service; most commercial ~90-180 days). Because a wrong-payer detour burns calendar time, correct and resend immediately.

Example

A front-desk keying error transposes two digits of the member ID, so the payer returns CO-31 (cannot identify as insured). Pulling the card, correcting the ID, and resubmitting clears it without any appeal.

Prevent CO-31 going forward

Scan insurance cards and run electronic eligibility before the date of service
Validate member ID, name, and DOB against the payer record at registration
Re-verify coverage at every visit, not just the first
Confirm the correct payer/plan when a patient mentions a coverage change

Code families most affected

All claim types (identity/eligibility is service-agnostic)
Patients with recent plan changes or multiple coverages
Claims routed to the wrong payer or plan

Related codes

Denial codes you'll often see alongside CO-31

CO-16 CO-109 CO-22

Payer notes

CARC 31 is an identity/eligibility mismatch and can appear under either CO or PR depending on the payer. Verify the patient's coverage before treating any balance as the patient's — the usual cause is a fixable data error, not true non-coverage.

Fix this CO-31 denial the right way

CO-31 is resolved with a corrected claim, not an appeal. Ask D3 gives you the exact correction steps — free, backed by CMS, Medicare, and major-payer data.

See CO-31 correction steps Ask D3 about CO-31 All denial codes

Medical billing disclaimer

CARC/RARC definitions are the standardized X12 set; group-code semantics follow the X12 Claim Adjustment Group Code standard. Filing and appeal windows vary by payer — the standard anchors shown (Medicare redetermination 120 days from the remittance; Medicare timely filing 12 months from date of service; most commercial payers ~90-180 days) are general references, not a guarantee for any specific plan. Always confirm the rule in the payer's provider manual before acting. D3rx is not responsible for claim outcomes.