Evaluation of patch test results by use of the reaction index. An analysis of data recorded by the Information Network of Departments of Dermatology (IVDK)*

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Abstract
Results of patch tests performed in 15,553 patients by 18 dermatological departments (members of the German Contact Dermatitis Research Group) and recorded by the Information Network of Departments of Dermatology (IVDK) were analysed by comparing recently defined reaction indices (RIs). The RIs studied were calculated from the numbers of allergic (a), questionable (q), and irritant (i) reactions (RI = (a-q-i/(a + q + i)), which were obtained using 13 European standard allergens. RIs were calculated for all patients and for separate subgroups defined by age, sex, history of atopic dermatitis, time of allergen exposure (1 versus 2 days), and time of patch test reading (2 or 3 days after allergen application). Higher RIs were consistently obtained when patch tests were applied for 1 day, as compared to 2 days. Readings at 3 days after allergen application resulted in higher RIs than readings after 2 days. In contrast, sex, age, and history of atopic dermatitis of patients were not found to have a consistent influence on the RIs. We suggest that reading after 3 days should be obligatory, and that allergen exposure for 1 day instead of 2 days might make patch test evaluation easier. These suggestions need to be substantiated by data on clinical relevance.