Photopatch Testing

Abstract

Photopatch testing is a procedure that combines patch testing with UVirradiation. It is primarily indicated to study photoallergic contact dermatitis, but it can also contribute to establish a correct diagnosis of any exposed site dermatitis or systemic drug photosensitivity. Photopatch testing has been standardized with the input of the European Society of Contact Dermatitis and European Society of Photodermatology, but it is still seldom performed. It consists on the application of two sets of allergens for 1 or 2 days, irradiation of one set at D1 or D2 with 5 J/cm2 of UVA, and readings immediately after irradiation and two or more days thereafter. A European photopatch test baseline and extended series of allergens, including mostly UV filters and topical drugs, particularly NSAIDs, were recommended in 2013. Chemicals causing photoallergy suffer temporal and regional variations, and “historical” photosensitizers not marked in Europe may also be occasionally responsible. Therefore, apart from these series, patients’ own products or other chemicals to which the patient was exposed to should also be considered for testing. Interpretation of the results is essential as reactions can represent contact allergy, photoallergy, photo-augmented allergic contact dermatitis, or nonspecific phototoxic or irritant reactions.