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 it is not serious 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 of positive reaction	Encountered more often with gold.
Pigmentary alteration	This includes postinflammatory hyper- and hypopigmentation.
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.

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