Workshop report Photopatch testing - methods and indications

Summary This workshop report aims to provide a concise and up to date summary of the clinical use and methodology of photopatch testing. Specific recommendations are made concerning a standard series of six potential photoallergens. Photoallergic dermatitis reactions are considered to be a form of cell-mediated hypersensitivity in which a photoactivated chemical or a photoproduct acts as a hapten or a complete antigen. Ultraviolet exposure is required both for induction and elicitation of the immune response. In contrast, phototoxic (or ‘photoirritant’) reactions are not considered to be immunologically mediated, and will occur in all subjects given sufficient exposure to the photosensitizer and ultraviolet radiation. The acute reactions to porphyrins and psoralens are examples of phototoxicity. Some compounds, for example chlorpromazine, can induce both phototoxic and photoallergic reactions. A further complication is that many photoallergens can, in some subjects, also induce allergic contact reactions without ultraviolet activation. Photopatch testing is used to diagnose and investigate photoallergic contact dermatitis. Other than confirming that a substance has photosensitizing potential, photopatch testing is not useful for suspected phototoxic skin reactions. There are widespread differences in the practice of photopatch testing, both within the U.K. and in other countries. This is a report of a British Photodermatology Group workshop on photopatch testing, held in Newcastle upon Tyne in November 1995. The main purpose of the workshop was to discuss the indications, technique and interpretation of results of photopatch testing, in order to provide a list of photoallergens of current clinical relevance and to give guidance on technique.