CO-31: patient cannot be identified as our insured.

What X12 says CARC 31 means

Patient cannot be identified as our insured.

CARC 31 · Start 01/01/1995, last modified 09/30/2007.

In plain terms: The payer cannot match the patient to a member on its files using the information on the claim.

What it means on a remittance

Payers match a claim to a member using the ID, name, date of birth and relationship fields. If those do not line up with a single member, the claim cannot be assigned. The remittance often carries a remark code for the patient identifier, such as N382. Where the payer treats the claim as unprocessable, the remark MA130 says no appeal rights are afforded and a new, corrected claim is to be submitted.

Typical causes

• A digit or letter wrong in the member ID, or the ID prefix or suffix left off.
• The patient is a dependent but the claim lists them as the subscriber, or the reverse.
• Name spelled differently from the card, or date of birth wrong.
• The patient's coverage ended or changed, so the old ID no longer matches.

These are common billing patterns, not a diagnosis of your claim. The remittance and the payer's policy decide.

What to check

1. Compare the claim with a copy of the patient's current card, front and back.
2. Check the subscriber's and the patient's names, dates of birth and relationship code.
3. Run an eligibility check with the same details you plan to bill.
4. Check which payer the claim went to against the name on the card.

Correct, resubmit or appeal

Correct the identifier fields and submit a corrected or new claim as the payer directs. The remark code decides the path: with MA130 there are no appeal rights and a new claim is requested. Keep the filing deadline in view: Medicare allows one calendar year after the date of service (42 CFR 424.44), and the calculator on this page shows the date. A corrected claim sent after the deadline can come back CARC 29.

Medicare dates that apply

Medicare claims must be filed no later than the close of the period ending 1 calendar year after the date of service (42 CFR 424.44(a)(1)), with exceptions in 424.44(b). A redetermination request must be filed within 120 calendar days from the date a party receives the notice, receipt is presumed 5 days after the notice date (42 CFR 405.942(a)), and a late request can be considered for good cause (405.942(b)). Clerical errors, including inaccurate data entry, are handled by the contractor as reopenings instead of redeterminations (42 CFR 405.980(a)(3)). The tool above runs the date math.

Related codes

X12 does not publish a table that pairs each CARC with remark codes; the payer picks the remark code and prints it beside the CARC on the same remittance line. These X12 remark codes carry wording that speaks to this reason, so look for them:

• N382 — Missing/incomplete/invalid patient identifier.
• MA130 — Your claim contains incomplete and/or invalid information, and no appeal rights are afforded because the claim is unprocessable. Please submit a new claim with the complete/correct information.

Other codes: CARC 16 (missing or wrong claim data), CARC 29 (filing deadline expired). The full lookup reads any code on this page.

Worked example (hypothetical)

A claim for a dependent lists the subscriber's ID with no suffix and the patient's own birth date. It comes back CO-31. The biller uses the card to add the dependent's suffix, runs eligibility, and resubmits.

Sources

• X12: Claim Adjustment Reason Codes — code text, usage notes and dates (list dated 10/30/2026, retrieved 2026-11-02)
• X12: Remittance Advice Remark Codes — remark code text (retrieved 2026-11-02)
• 42 CFR 424.44, 42 CFR 405.942 and 42 CFR 405.980 — eCFR, current through 2026-10-29 (retrieved 2026-11-02)

Code descriptions are X12 copyrighted material, quoted here only for the codes on this page and linked to the owner. For the full lists, use X12. Every rule above was read from these pages on November 2, 2026. Payers revise code lists, contracts and policies, so confirm anything that affects a claim or a deadline with the payer.