Background: Cyclic acid anhydrides often cause allergic respiratory diseases, but contact urticaria because of these anhydrides has been considered rare.
Objective: We describe 21 patients diagnosed with occupational contact urticaria at the Finnish Institute of Occupational Health during the period 1990-2006.
Method: Prick test with human serum albumin (HSA)-acid anhydride conjugates, determination of specific immunoglobulin E (IgE), and open application were used in the diagnosis.
Results: The majority of the patients worked in the manufacture of electrical machines and were exposed to an epoxy hardener containing methyl hexahydrophthalic anhydride. The largest prick test reaction was often noted for the acid anhydride the patient had been exposed to. The specific IgE results were mostly in line with the prick test reactions. Phthalic anhydride IgE was determined in 20 patients and was found positive in 19 patients. In open application, a positive test result often required the use of the undiluted hardener.
Conclusions: Contact urticaria may be more common than previously believed. Prick tests with HSA-acid anhydride conjugates were a useful test method for detecting immediate sensitization to acid anhydrides. The determination of the specific IgE gave almost equal results. The open application test often had to be performed with the undiluted anhydride product to obtain a positive test result.