Sensitization to methylisothiazolinone in a group of methylchloroisothiazolinone/methylisothiazolinone allergic patients.

In the EU, Methylisothiazolinone (MI) was approved as a preservative in cosmetics and household products in 2005. Since then, several cases of MI contact allergy have been reported. We studied the prevalence of MI sensitization in patients allergic to Methylchloroisothiazolinone/Methylisothiazolinone (MCI/MI). From a total of 751 patients submitted to patch testing between 2007 and 2011, 26 were sensitized to MCI/MI 100 ppm aq. Fifteen patients were retested with MCI/MI 100 ppm aq, MI 200 ppm aq and personal products, and the relevance of positive results, concerning the use of personal products, was evaluated. Seven of the 15 MCI/MI positive patients were positive to MI: six patients showed an equally intense reaction to MCI/MI and MI alone, and another one presented with a stronger reaction to the latter compound. All positive reactions were relevant. The use of MI alone as a preservative does not seem to be a safe alternative. In this study, the majority of positive reactions to MCI/MI and MI had similar intensity, although primary sensitization to MI has to be considered according to each individual case; moreover, primary sensitization to MCI with cross-reactivity to MI cannot be excluded. Further studies are needed to determine the optimal concentration for MI patch testing.