Denial codes · Denial code
CO-31 on a nutrition claim: the payer can't find your patient
By Javier Alvarez · Updated · 4 min read
| Official description | Patient cannot be identified as our insured. |
|---|---|
| Group code | CO: do not bill the patient for a data mismatch |
| Common remark codes | N382 (patient identifier), MA36 (patient name), MA27, N30 |
| Top MNT causes | BCBS prefix dropped, dependent billed as subscriber, name or DOB typo |
| Medicare | Use the 11-character Medicare Beneficiary Identifier (MBI) from the card |
| Fix route | Corrected or new claim with matching data; not an appeal |
CO-31 means the payer looked for your patient in its member file and came up empty. Nothing was decided about the visit or about coverage. Somewhere between the insurance card and the claim, a character, a name or a relationship changed, and the payer's matching failed. In our experience with dietitian claims, the fix is almost always in the first 30 seconds of comparing the claim to a photo of the card.
What CO-31 means (official X12 wording)
The X12 Claim Adjustment Reason Code list defines CARC 31 as:
"Patient cannot be identified as our insured."
It often comes with remark code N382, "Missing/incomplete/invalid patient identifier." Other remark codes point at a specific field: MA36, "Missing/incomplete/invalid patient name," and, on Medicare remits, MA27, "Missing/incomplete/invalid entitlement number or name shown on the claim." Some payers send N30, "Patient ineligible for this service." Treat all of them the same way: the identity data on the claim does not match the payer's file.
Which claim fields the payer matches
On the CMS-1500 (and the equivalent 837P loops), payers match on some combination of:
| Field | CMS-1500 item | What goes wrong |
|---|---|---|
| Insured's ID number | 1a | Missing prefix or suffix, a digit transposed, an old card |
| Patient's name | 2 | Nickname or married name instead of the name on the card |
| Patient's birth date | 3 | Typo, or the subscriber's DOB in the patient's field |
| Insured's name | 4 | Patient listed as the insured when a parent or spouse is the subscriber |
| Patient relationship to insured | 6 | "Self" for a dependent |
| Insured's address and other fields | 7, 11 | Rarely the cause, but some plans use them |
The NUCC 1500 instructions define each item; our CMS-1500 guide for dietitians walks through them in MNT terms.
The usual causes on nutrition claims
1. Blue Cross member ID without its prefix
Blue Cross Blue Shield IDs usually start with a three-character alpha prefix that routes the claim to the member's home plan. Drop it, or type a zero for an "O", and the claim cannot be matched. This is doubly important for out-of-state Blue members filed through your local plan under BlueCard; see BlueCard for out-of-state Blue members.
2. A dependent billed as the subscriber
A teenager with an eating disorder or a child with type 1 diabetes is usually a dependent on a parent's plan. If the claim lists the child as "self" with the child's name in the insured fields, the plan finds no subscriber by that name. The parent goes in Item 4, the child in Item 2, relationship in Item 6.
3. The name on the claim is not the name on the card
Patients book with the name they go by. Your intake form, your practice software or a scheduling tool may carry "Kate" when the plan has "Katherine", or a married name the plan has not updated. The claim must use the legal name exactly as the payer has it.
4. Old Medicare numbers
Medicare replaced Social Security-based numbers with the Medicare Beneficiary Identifier (MBI), an 11-character ID printed on the current card (CMS). A patient who hands you an old card, or a Medicare Advantage card when you are billing Original Medicare (or the reverse), creates a mismatch.
5. The patient is not actually that plan's member
The card is from a previous employer, the patient gave you a spouse's card for a plan they are not on, or the plan was never activated. That is not a data typo: you need the correct payer. If the plan finds the member but the coverage ended, the code is usually CO-27 instead.
How to fix a CO-31
- Pull the card photo and an eligibility response. Compare the claim to both, field by field: ID with prefix and suffix, name spelling, date of birth, subscriber, relationship.
- Call the payer if you cannot see the difference. Ask what it has on file for the member (payers will confirm when you give the member ID and DOB) and note the call reference.
- Resubmit with matching data. Ask the payer which route it wants: many treat an unmatched claim as unprocessed and want a new claim; others take a corrected claim (frequency code 7 with the original claim number). If the right payer turns out to be a different one, send a new claim there.
- Fix it in the patient record, not only on this claim, so every future claim for the patient is right.
- Do not appeal. There is no coverage decision to contest.
How to prevent CO-31
- Photograph both sides of the card at intake and at every plan change, and copy the ID character by character, including letters.
- Record the subscriber separately from the patient for every dependent, with the subscriber's name and date of birth exactly as the plan has them.
- Run an eligibility check before the first visit. If the 271 comes back with a match, the identifiers you used are the ones that work. If it does not, fix them before you see the patient.
- Keep the legal name and the preferred name in separate fields, and bill with the legal name.
- Re-verify each new plan year, when many plans issue new cards and IDs.
CO-31 is a close cousin of CO-16, which some payers use with N382 for the same member ID problem. For every other code, see the denial code lookup.