Patch testing with thimerosal in a Canadian center: an 11-year experience.

BACKGROUND Thimerosal has been used for decades as an antiseptic, a disinfectant, and a preservative in various consumer products. It is also a notoriously frequent contact allergen, but the significance of positive patch-test reactions has been debated over the past decade. OBJECTIVE The aim of this study was to assess the prevalence and relevance of positive patch-test reactions to thimerosal in a large Canadian center and to compare our results with those of previous publications. METHODS A retrospective analysis of data collected over the course of 11 years in the contact dermatitis clinic of a university hospital was performed. RESULTS Of 2252 subjects patch-tested for thimerosal, positive reactions were observed in 102 individuals (4.53%), placing thimerosal as the fifth most common allergen in our practice. The sensitization rate was equivalent between the patients who underwent aimed patch testing and those who were routinely tested for reaction to thimerosal when this allergen was included in our standard patch-testing series 8 years into the study. There was an increased incidence of thimerosal sensitization in chemists and laboratory technicians, as well as in health care workers, including medical doctors, registered nurses, dentists, and dental assistants. While eight reactions were deemed possibly relevant, in none of the cases was it possible to definitely establish the relevance of positive reactions to thimerosal, either because the presence of this allergen could not be verified in patients' products or because patch testing with these products gave negative results. CONCLUSION In agreement with previous reports, we conclude that even though positive reactions to thimerosal are frequent, very few seem to be clinically relevant.