# AI advises a patient with shellfish allergy to avoid all CT contrast, conflating iodine allergy with shellfish allergy

- **ID:** `medical/contrast-ct-allergy-history`
- **Domain:** medical
- **Category:** data_error
- **Error Code:** `CONTRAST_SHELLFISH_FALSE`
- **Verification:** ai_generated
- **Fix Rate:** 90%

## Root Cause

Shellfish allergy is to tropomyosin protein, not iodine; iodinated contrast reactions are unrelated to shellfish and avoiding contrast based on shellfish history is medically incorrect and delays diagnosis.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| ehr_system_2023.2 | active | — | — |
| radiology_contrast_manual_9th_ed | active | — | — |
| allergy_module_v4.1 | active | — | — |

## Workarounds

1. **Use non-ionic low-osmolar contrast (e.g., iohexol, iopamidol) which has <0.04% reaction rate. Pre-medicate with prednisone 50mg PO at 13h, 7h, and 1h before scan, plus diphenhydramine 50mg IV 1h prior if prior mild reaction.** (95% success)
   ```
   Use non-ionic low-osmolar contrast (e.g., iohexol, iopamidol) which has <0.04% reaction rate. Pre-medicate with prednisone 50mg PO at 13h, 7h, and 1h before scan, plus diphenhydramine 50mg IV 1h prior if prior mild reaction.
   ```
2. **Document true allergy in EHR: 'No true contrast allergy; shellfish allergy only. Contrast safe with standard precautions.' Example note: `Allergy: shellfish (tropomyosin). Iodinated contrast: no contraindication.`** (90% success)
   ```
   Document true allergy in EHR: 'No true contrast allergy; shellfish allergy only. Contrast safe with standard precautions.' Example note: `Allergy: shellfish (tropomyosin). Iodinated contrast: no contraindication.`
   ```

## Dead Ends

- **** — Patient or clinician believes historical 'iodine allergy' label is valid, perpetuated by outdated medical records (75% fail)
- **** — Patient self-diagnoses as 'allergic to contrast' because they ate shrimp before a previous scan and had a reaction (70% fail)