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We have studied the influence of variations in allergen concentration and UVA dosaging on the results of photopatch testing with the Scandinavian standard photopatch series in 29 patients with photocontact and/or contact allergy to 1 or several of the allergens in that series. Photocontact test reactions were more sensitive to allergen dilution than plain contact test reactions. Even dilution from the standard 5% to 2.5% significantly reduced para-aminobenzoic acid photocontact test reactions. Reducing the UVA dose from the standard 5 J/cm2 to 2.5 or 1 J/cm2 in 2 out of 5 cases turned a significant (++) reaction into a doubtful one (+). Increasing the standard UVA dose of 5 J/ cm2 to 20-40 J/cm2 turned a single + photocontact reaction to trichlorcarbanilide and a single 1 + plain contact reaction to chlorhexidine into ++ reactions. In the majority of cases, however, neither photocontact nor plain contact test reactions were augemented by UVA doses up to 80 J/cm2. We conclude that a UVA dose of 5 J/cm2 is sufficient for eliciting photocontact allergic test reactions, and that a reduction of either the UVA dose level or the standard allergen concentrations of the Scandinavian photopatch test guidelines may cause loss of significant photocontact test reactions in a proportion of the cases.

The possible modulating effect of previous nickel sensitization on subsequent cobalt sensitization, and vice versa, was studied in a guinea pig model, using an open epicutanous induction protocol. Challenge tests were made by both topical and systemic routes. Controls included animals sensitized to only one of the metals. Animals sensitized with both metals seemed to react more readily in patch testing to either allergen, as compared to mono-sensitized animals. Systemic challenge with cobalt revealed a significantly (p less than 0.05) higher reaction rate (70%) in animals originally sensitized to cobalt and subsequently to nickel, as compared to the reaction rate (20%) in cobalt mono-sensitized animals. The reverse order of double-sensitization (nickel first, cobalt subsequently) led to an intermediate cobalt sensitization rate (50%). These experimental data corroborate clinical findings which have indicated a mutual enhancing effect of nickel and cobalt contact sensitization in man, and provide an animal model for studying the underlying immunological processes.