Contact photoallergy testing of sunscreens in guinea pigs

The patient group consisted of 18 elderly male patients with persistent light reactivity who were subjected to extensive phototesting with different wavelengths, including patch and photopatch testing. All reacted adversely to ultraviolet light and some also to longer wavelengths when tested on normal appearing skin. 17 patients showed contact or photocontact reactions, of which 12 were positive photopatch test reactions and 11 were plain contact reactions. Contact allergy to constituents of oak moss and different lichen compounds was twice as common as allergy to Compositae oleoresins. 72 patients with chronic polymorphic light eruption were used as a control group. 10 of these patients had either a positive photopatch test reaction or a plain contact allergy. Patients with persistent light reactivity are characterized by a particular susceptibility to develop a delayed-type hypersensitivity. They frequently have both photo and plain contact allergy, often to substances used in cosmetics. In 13 of 16 patients in whom a biopsy was carried out, the histology supported the clinical diagnosis. In none of the biopsies was the picture diagnostic in itself. This underlines the inadequacy of light microscopy as the only diagnostic procedure.