Patch testing is performed for allergic contact dermatitis. A patient shows 2+ reaction at 72 hours to fragrance mix but not at 48 hours. The interpretation of this 'late positive' reaction is:
- A Irritant reaction — should be ignored
- B Non-specific false positive due to tape occlusion
- C True delayed hypersensitivity — clinically relevant allergy to fragrance ✓
- D Immediate IgE-mediated reaction to fragrance allergens
Explanation
In patch testing, readings at 48 and 96 hours (or 72 and 96 hours) are standard. A reaction appearing or strengthening at 72–96 hours (late positive) that was absent at 48 hours is a TRUE Type IV (delayed-type) hypersensitivity reaction — indicating genuine allergic sensitisation. This pattern is particularly seen with metals (gold, neomycin, corticosteroids, and fragrance mixes). An irritant reaction typically peaks early (48 h) and fades — it does NOT increase at 96 h (the 'decrescendo pattern'). Allergic reactions show an 'crescendo pattern' — increasing over time.
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.