Common shoe allergens undetected by commercial patch-testing kits: dithiodimorpholine and isocyanates.

The diagnosis of allergic contact dermatitis of the feet is challenging because the constituents of shoes are not labeled. In addition, the materials and technologies used in the manufacturing of shoes are continuously evolving. Since the 1950s, rubber allergens have been the most common cause of shoe dermatitis. However, the causal allergens in rubber have changed. Instead of 2-mercaptobenzothiazole, dithiodimorpholine, an allergen that is not contained on standard patch-testing trays, now may be one of the more common rubber sensitizers in shoes. Other allergens not found on standard series that can account for shoe dermatitis include isophorone diisocyanate, dimethylaminoethyl ether, and mixed dialkyl thioureas. Therefore, physicians evaluating patients with allergic-appearing foot dermatitis need to test with allergens beyond those present on standard screening series.