# CARC 4 Denial Code (Procedure Code Inconsistent with Modifier Used) | ANSI 835 EDI Spec
> Comprehensive healthcare EDI technical reference and denial resolution guide for Claim Adjustment Reason Code 4 (Procedure Code Inconsistent with Modifier Used) under ANSI ASC X12 835 005010X221A1.

- **Canonical URL:** https://stanzaapi.com/tools/x12-parser/carc/carc-4-modifier-inconsistent
- **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) 4 communicates: "The procedure code is inconsistent with the modifier used or a required modifier is missing.".

In ANSI X12 835 Electronic Remittance Advice (ERA) files, CARC 4 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*4*AMOUNT~.

Root Cause: Incompatible CPT/HCPCS modifier (e.g., billing bilateral modifier 50 on a code that already includes bilateral services, or missing modifier 25 on E/M with procedure).

Adjudication & Appeal Strategy: Review CPT modifier rules, remove invalid modifier or append missing anatomical/informational modifier, and submit corrected claim.

## Specification Matrix
| Attribute | Value | Technical Guidance |
| --- | --- | --- |
| Reason Code | CARC 4 | Procedure Code Inconsistent with Modifier Used |
| Standard Group Code | CO | Contractual Obligation |
| ANSI 835 Segment | CAS*CO*4*AMOUNT~ | Loop 2110 or Loop 2100 |
| Balance Billing Patient | Prohibited. | Compliance with billing regulations |
| Common RARCs | M20, N519 | Remittance Advice Remark Codes |
| Recommended Action | Review CPT modifier rules, remove invalid modifier or append missing anatomical/informational modifier, and submit corrected claim. | 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": "4",
        "reasonDescription": "Procedure Code Inconsistent with Modifier Used",
        "adjustmentAmount": 500
      }
    ]
  }
}
```

## Frequently Asked Questions & Edge Cases
### Q: What does CARC 4 mean on an 835 ERA?
**A:** CARC 4 signifies: "The procedure code is inconsistent with the modifier used or a required modifier is missing.". It explains why the payer did not pay the full billed charge.

### Q: Can a provider balance-bill the patient for CARC 4?
**A:** Prohibited.

### Q: How do medical billing teams resolve and appeal CARC 4?
**A:** Review CPT modifier rules, remove invalid modifier or append missing anatomical/informational modifier, and submit corrected claim.