# CARC 31 Denial Code (Patient Cannot Be Identified as Insured) | ANSI 835 EDI Spec
> Comprehensive healthcare EDI technical reference and denial resolution guide for Claim Adjustment Reason Code 31 (Patient Cannot Be Identified as Insured) under ANSI ASC X12 835 005010X221A1.

- **Canonical URL:** https://stanzaapi.com/tools/x12-parser/carc/carc-31-patient-cannot-be-identified
- **Governing Standard:** ANSI ASC X12 Standards (005010) (Accredited Standards Committee X12 / HIPAA 5010)
- **Edge Latency SLA:** <2ms Edge (Cloudflare V8 Isolates)
- **Execution Model:** Pure compute $f(x) = y$, 100% In-Memory (Zero Store, Zero Log)

## Technical Overview & Governing Rules
Claim Adjustment Reason Code (CARC) 31 communicates: "Patient cannot be identified as our insured.".

In ANSI X12 835 Electronic Remittance Advice (ERA) files, CARC 31 is reported in Loop 2110 (Service Payment Information) or Loop 2100 (Claim Payment Information) under the Claim Adjustment (CAS) segment with Group Code 'CO' (Contractual Obligation), formatted as: CAS*CO*31*AMOUNT~.

Root Cause: Subscriber ID number does not exist in payer database (often wrong payer ID or switched health plan).

Adjudication & Appeal Strategy: Contact patient to verify current insurance card and Payer ID. Re-route 837 claim to correct clearinghouse Payer ID.

## Specification Matrix
| Attribute | Value | Technical Guidance |
| --- | --- | --- |
| Reason Code | CARC 31 | Patient Cannot Be Identified as Insured |
| Standard Group Code | CO | Contractual Obligation |
| ANSI 835 Segment | CAS*CO*31*AMOUNT~ | Loop 2110 or Loop 2100 |
| Balance Billing Patient | Prohibited until insurance eligibility is verified. | Compliance with billing regulations |
| Common RARCs | MA61, N30 | Remittance Advice Remark Codes |
| Recommended Action | Contact patient to verify current insurance card and Payer ID. Re-route 837 claim to correct clearinghouse Payer ID. | Workflow for billing teams |

## Standard JSON Response Model
```json
{
  "transactionType": "835",
  "claimNumber": "CLM-2026-001",
  "adjudication": {
    "billedAmount": 500,
    "paidAmount": 0,
    "adjustments": [
      {
        "groupCode": "CO",
        "groupName": "Contractual Obligation",
        "reasonCode": "31",
        "reasonDescription": "Patient Cannot Be Identified as Insured",
        "adjustmentAmount": 500
      }
    ]
  }
}
```

## Frequently Asked Questions & Edge Cases
### Q: What does CARC 31 mean on an 835 ERA?
**A:** CARC 31 signifies: "Patient cannot be identified as our insured.". It explains why the payer did not pay the full billed charge.

### Q: Can a provider balance-bill the patient for CARC 31?
**A:** Prohibited until insurance eligibility is verified.

### Q: How do medical billing teams resolve and appeal CARC 31?
**A:** Contact patient to verify current insurance card and Payer ID. Re-route 837 claim to correct clearinghouse Payer ID.