Patch‐Test Reactions to Thioureas Are Frequently Relevant

Background: Thioureas are used as antioxidants in the manufacture of rubber (especially neoprene) and as fixatives in photography and photocopy paper. The number of reported cases of allergic contact dermatitis from thioureas is relatively low, and standard patch test series often do not contain thiourea allergens. Thioureas are included in our institution's standard patch test series. Objective: To examine the occurrence and relevance of patch‐test reactions to thioureas. Methods: We retrospectively analyzed patch‐test data for patients evaluated between January 1999 and June 2001. Patients included in the study underwent testing with a standard screening series of allergens and a standardized patch testing technique. Our standard patch test series includes mixed dialkyl thioureas at a concentration of 1% in petrolatum. Results: Of 1,368 patients tested with our standard series, 33 patients (2.4%) had patch‐test reactions to mixed dialkyl thioureas at day 5. Reactions in 14 (42%) of those patients were of definite clinical relevance, indicating a confirmed exposure to thioureas in many different settings. Conclusion: Patch‐test reactions to thioureas occur as often as do reactions to many other allergens in the standard series and are often relevant. Institutions should consider including mixed dialkyl thioureas in their standard patch test screening series.