Background: The preservative methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) is a well-known contact sensitizer. Historically, there have been different opinions on the optimal patch test concentration of MCI/MI, and both 0.01% and 0.02% aqueous (aq.) have been proposed. In 2011, based on literature reviews, it was recommended that the concentration of 0.02% aq. should be used in the international baseline series. Objectives: The aim of this study was to verify the recommendation from 2011 by comparing the patch test results from consecutive patch testing with MCI/MI 0.01% and 0.02% in clinics representing countries around the world. Patients and Methods: Two thousand seven hundred three consecutive patients with dermatitis in 8 dermatology clinics representing 8 countries were patch tested with MCI/MI 0.01% aq. and, in parallel with MCI/MI 0.02% aq., provisionally included in the baseline series. Results: Contact allergy to MCI/MI at 0.01% and 0.02% was found in 3.7% and 5.6% of the patients, respectively (P G 0.001). Conclusions: Methylchloroisothiazolinone/MI 0.02% aq. (dose, 6 Kg/cm2) diagnoses significantly more contact allergy than 0.01% (dose, 3 Kg/cm2), without resulting in more adverse reactions.Methylchloroisothiazolinone/MI at 0.02% aq. should therefore be continuously used in the international baseline series.
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