Positive patch-test reactions to mixed dialkyl thioureas: Cross-sectional data from the North American Contact Dermatitis Group, 1994 to 2004

Background: Allergic contact dermatitis from thioureas is uncommon but may result from exposure to rubber, especially neoprene. Objectives: To (1) describe the population with positive patch-test reactions to mixed dialkyl thioureas (MDTU) (ie, diethylthiourea and dibutylthiourea); (2) determine clinical and occupational relevance associated with reactions to MDTU and identify the most commonly related sources and occupations; and (3) examine the frequency of co-reacting allergens in MDTU-positive patients. Methods: A retrospective analysis of cross-sectional data of 22,025 patients patch-tested by the North American Contact Dermatitis Group between 1994 and 2004. Results: Of 21,898 patients tested with MDTU, 225 (1.0%) had positive reactions; of these, 173 (76.9%) were currently relevant and 29 (17.1%) were occupationally relevant. Patients positive to MDTU were 2.6 times more likely to have foot involvement than patients with positive reactions to other allergens (p < .0001). Footwear was the most commonly identified source overall (20.0%) whereas gloves were the most common occupational source. Of the 173 patients with currently relevant MDTU reactions, 24.9% also reacted to another rubber allergen. Conclusions: Current clinical relevance of reactions to MDTU was high; occupational relevance was less frequent. Patch tests with common rubber allergens (carbamates, thiurams, and mercaptobenzothiazole) may fail to detect many cases of thiourea-induced rubber allergic contact dermatitis.