Patch Testing
Patch testing is the gold standard diagnostic procedure used to identify Type IV (Delayed-Type) Hypersensitivity reactions in patients with suspected Allergic Contact Dermatitis (ACD).
Procedure & Interpretation
- Method: Suspected allergens are applied to the patient’s back under hypoallergenic tape/chambers for 48 hours.
- Timing: Results are read at 48 hours (after patch removal) and again at 72–96 hours to capture delayed responses.
- Clinical Correlation: Positive results must be relevant to the patient’s exposure history to be considered the causative agent.
Common Contact Allergens
| Allergen | Typical Source |
|---|---|
| Nickel | Jewelry, belt buckles, watch straps. |
| Fragrance Mix | Personal care products, perfumes. |
| Neomycin | Topical antibiotic ointments. |
| Balsam of Peru | Flavorings, dental products, fragrances. |
High-Yield Exam Pearl:
- Allergic (ACD) vs. Irritant (ICD): ACD is immune-mediated (Type IV hypersensitivity), whereas ICD is a direct cytotoxic effect. ACD typically presents with pruritus; ICD presents with burning/stinging.
- Nickel Allergy: Often presents as dermatitis at the site of metal contact, such as the periumbilical area from a jean button or earlobes from earrings.