Patch testing for allergic contact dermatitis is performed with standardised concentrations in petrolatum. The reading is done at 48 hours AND 96 hours (4 days). Why is the 96-hour reading mandatory in addition to 48-hour reading?
- A Irritant reactions peak at 48 hours and reduce by 96 hours; allergic reactions may worsen
- B Some allergens (especially metals like nickel, gold, neomycin) cause late-reading reactions that appear only at 96 hours or beyond, and would be missed at 48 hours alone ✓
- C Petrolatum vehicle reactions require 96 hours to distinguish from true allergy
- D Systemic corticosteroids given before testing require 96 hours to clear
Correct answer: B. Some allergens (especially metals like nickel, gold, neomycin) cause late-reading reactions that appear only at 96 hours or beyond, and would be missed at 48 hours alone
Explanation
Certain contact allergens, particularly metals (nickel, gold, cobalt), neomycin, and corticosteroids, produce delayed-positive patch test reactions that may be absent or weakly positive at 48 hours but become clearly positive at 96 hours (or 7 days). Reading only at 48 hours would miss these late reactors. International Contact Dermatitis Research Group (ICDRG) guidelines mandate readings at D2 and D4; gold and corticosteroids may need D7. Irritant reactions typically peak early and fade.
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
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