Clarification and Modification of the International Contact Dermatitis Research Group Classification of Patch Test Reactions on Behalf of the International Contact Dermatitis Research Group.

The first ICDRG (International Contact Dermatitis Research Group) classification of patch test reactions was published in 1970. With this classification, many weak reactions were classified as allergic reactions. A second classification was therefore implemented one decade later on behalf of the ICDRG and the North American Contact Dermatitis Group. This classification was adopted by the European Society of Contact Dermatitis. Evaluation of the significance of the results of various studies on patch test reading demonstrated that a new ICDRG classification of patch test reactions was needed. A classification system should not be based on the exclusive presence/absence of papules and vesicles, i.e., the words "possibly papules/vesicles" allow for the unavoidable variation in reading among experts. Dermatologists are not reading patch test reactions in a uniform way due to lack of a clear definition of the threshold between weak allergic and doubtful reactions. A new ICDRG classification with clarifications and modifications of the old one is presented.