A 35-year-old man with a history of asthma presents with pruritus, erythema, and vesicles on his forearms after contact with nickel-plated jewelry. The lesions appear 48 hours after exposure. Which investigation is most appropriate to confirm the diagnosis of contact dermatitis (Type IV hypersensitivity)?

## Diagnosis of Type IV Hypersensitivity (Contact Dermatitis) ### Pathophysiology and Timeline **Key Point:** Type IV hypersensitivity is a T-cell–mediated delayed reaction that develops 24–72 hours after antigen exposure. The 48-hour presentation in this case is classic for contact dermatitis. **High-Yield:** Patch testing is the **gold standard** for confirming Type IV hypersensitivity and identifying the specific allergen. It reproduces the clinical reaction under controlled conditions. ### Patch Test Protocol | Feature | Detail | |---|---| | **Timing of application** | Applied to clean, uninvolved skin (usually back) | | **Reading times** | 48 hours (D2) and 96 hours (D4) | | **Positive reaction** | Erythema, induration, vesicles, or bullae at 48 h or 96 h | | **Mechanism** | Reproduces sensitized T-cell response to hapten-protein complex | | **Allergen panel** | Standard series includes nickel sulfate, chromium, formaldehyde, fragrance, etc. | **Clinical Pearl:** The patch test is performed in a **quiescent state** — the patient should not have active dermatitis during testing, as this may confound results. A positive patch test at 48 hours (or 96 hours) confirms sensitization and causality. ### Why Patch Test is Superior for Type IV Hypersensitivity 1. **Reproduces the reaction:** Directly challenges the sensitized T-cell population 2. **Identifies the allergen:** Allows testing against a standardized panel 3. **Delayed kinetics:** Matches the 24–72-hour timeline of Type IV reactions 4. **High specificity:** Positive patch test = clinically relevant sensitization ### Comparison of Investigations in Hypersensitivity | Investigation | Type I | Type II | Type III | Type IV | Notes | |---|---|---|---|---|---| | **Serum IgE / specific IgE** | ✓✓ | ✗ | ✗ | ✗ | Antibody-based; not relevant to T-cell reactions | | **Skin prick test** | ✓✓ | ✗ | ✗ | ✗ | Immediate reaction; contraindicated in Type IV | | **Patch test** | ✗ | ✗ | ✗ | ✓✓ | **Gold standard for Type IV** | | **Histamine release assay** | ✓ | ✓ | ✗ | ✗ | Mast cell / basophil activation; not Type IV | | **Complement fixation** | ✗ | ✓ | ✓ | ✗ | Immune complex detection | **Warning:** Skin prick testing is **contraindicated** in Type IV hypersensitivity because it measures immediate (Type I) reactions, not delayed T-cell responses. It will not detect contact allergens. ### Mnemonic: **PATCH** for Type IV Testing - **P**atch test (gold standard) - **A**pplied to uninvolved skin - **T**-cell mediated (delayed) - **C**ontact allergen identification - **H**ypersensitivity Type IV [cite:Robbins 10e Ch 6; Harrison 21e Ch 317]