A healthcare worker develops hand eczema. Patch testing to the European baseline series is performed. A reaction showing erythema and vesicles at 96 hours (D4) but NOT at 48 hours (D2) indicates:
- A Late or delayed reading — true positive allergic contact dermatitis (crescendo reaction) ✓
- B Irritant contact reaction, not true allergy
- C False positive due to pressure (tape) effect
- D Decrescendo reaction consistent with irritant response
Correct answer: A. Late or delayed reading — true positive allergic contact dermatitis (crescendo reaction)
Explanation
In patch testing, readings at 48 h and 96 h (or 72 h) are standard. A crescendo reaction that worsens between D2 and D4 readings is characteristic of true allergic contact dermatitis (Type IV hypersensitivity) because the T-cell-mediated reaction amplifies over time as more sensitised T cells are recruited. A decrescendo reaction (positive at D2, clearing at D4) is more consistent with irritant contact dermatitis. Some allergens (e.g., metals, neomycin, corticosteroids) are specifically known for late readings.
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.