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 "allergic" 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.