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©2013 UpToDate ® Print Email Adverse reaction to patch testing Active sensitization A negative reaction is followed by a late positive reaction after 10 to 20 days "Angry back" or "excites skin syndrome" A few strong positive reactions cause a chain of multiple reactions to other patch tests that would otherwise be negative Flare of dermatitis Usually not a serious event and helps to validate the positive reaction Adhesive tape reaction Rare with the current acrylate based adhesive tapes Koebner phenomenon Can be seen in patients with active psoriasis and lichen planus [1] Persistence positive reaction Encountered more often with gold Pigmentary alteration This includes post inflammatory hyper- and hypo-pigmentation Pressure effect Imprint caused by pressure from the chamber or when testing a solid material Bacterial and viral infections Higher risk (bacterial) when testing to plant substances Necrosis/scar/keloid Risk of testing to unknown caustic substances that burn a hole in the skin [2] Sarcoidal reaction Has been described with metals including palladium Milia Anaphylactoid Extremely rare but potential risk with bacitracin, neomycin, and contact urticaria References: Weiss G, Shemer A, Trau H. The Koebner phenomenon; review of the literature. JEADV 2002; 16:241. Thijs L, Deraedt K, Goosens A. Granuloma possibly induced by palladium after ear piercing. Dermatitis 2008; 19:E26.