Patch Testing to Diphenylguanidine by the North American Contact Dermatitis Group (2013–2016)

Background/objectives Carba mix (CM, 3% petrolatum) contains 1,3-diphenylguanidine (DPG, 1%), zinc diethyldithiocarbamate (1%), and zinc dibutyldithiocarbamate (1%). Because DPG is a component of CM, DPG is often not tested separately. The purpose of this study was to determine the frequency of concomitant reactions to CM and DPG. Methods A retrospective analysis of the 2013-2016 North American Contact Dermatitis Group data was conducted. The study group consisted of patients with final interpretation of to either DPG or CM. Reactions coded as irritant or doubtful/macular erythema (+/- and not interpreted as allergic) were excluded. Results A total of 10,457 patients were patch tested to both CM and DPG, and 610 (5.8%) had allergic reactions to either CM or DPG (CM only [n = 292, 47.9%], DPG only [n = 190, 31.1%], both [n = 128, 21.0%]). A total of 39.4% of CM-allergic patients reacted to DPG, and 59.7% of DPG-allergic patients reacted to CM. Analyses found that 25% (++/+++ subgroup) to 40% (all patients) of allergic reactions to DPG would have been missed by testing to CM alone. More than 70% of reactions to CM and DPG were +/- or +. Conclusions Patch testing to CM will miss 25% to 40% of positive reactions to DPG. Both CM and DPG have a high frequency of +/- and + reactions.