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National report - The North American Contact Dermatitis Group (NACDG) has named mixed dialkyl thioureas as the 2009 Allergen of the Year, according to Bryan E. Anderson, M.D., associate professor of dermatology at Penn State College of Medicine in Hershey, Pa.
Primarily used as an accelerant in the manufacture of rubber and as an antioxidant in the manufacture of neoprene, thioureas are generally produced by passing liquid hydrogen sulfide though an aqueous bath of calcium cyanamide.
The resulting white crystalline material is a compound of hydrogen, oxygen, carbon and sulfur. The term "mixed dialkyl thioureas," or MDTU, refers to a combination of the two most prevalent compounds: diethylthiourea and dibutylthiourea.
"If you suspect a neoprene allergy, give the patient a MDTU patch test."
NACDG added MDTU to its standardized tray in 1991. Thioureas are not included in the European Standard Series or the True Test, but specialized rubber trays are widely available.
From 1998 to 2000, the NACDG patch-tested 5,807 patients and found a 1.1 percent positive reaction rate to MDTU. During roughly the same period, members of the Mayo Clinic's Contact Dermatitis Group found a 1.2 percent positive reaction rate among a test population of 1,321 patients.
"Any allergen, except possibly nickel, will have the same physical presentation," Dr. Anderson says.
"What differentiates thioureas from other allergens is a specialized geographic distribution."
The hands are the most common location for occupational exposures, due to the use of gloves that leach thioureas. Non-occupational exposure is more typically found in the feet, where it is caused by leaching of thioureas from foot supports or athletic shoes.
Physicians have also linked cases of allergic contact dermatitis to a wide variety of additional products, including rubber orthopedic braces, prostheses, splints, rubber masks, swim masks, goggles, wet suits and slimming suits, apnea masks and rubber-based materials in automobiles.
"To diagnose thioureas, you have to take a good history," Dr. Anderson says. "If a patient works in the neoprene or photographic industries, that raises suspicion of MDTU."
Patch-testing with MDTU picks up about 75 percent of relevant thiourea reactions. If there is a high probability that a thiourea might be the cause of a patient's dermatitis, the physician can patch test with a series of thiourea compounds to increase the percentage of relevant reactions.
Other thioureas available for patch-testing include dimethylthiourea, ethylbutylthiourea, diphenylthiourea and ethylenethiourea. With the exception of ethylenethiourea, there is a high degree of cross-reactivity among these compounds.