Patch Testing With Methylchloroisothiazolinone/Methylisothiazolinone Using a New Diagnostic Mix-A Multicenter Study From the International Contact Dermatitis Research Group

BACKGROUND: In the early 1980s, a preservative containing a mixture of methylchloroisothiazolinone (MCI) and methylisothiazolinone (MI) in a ratio of 3:1 was introduced. This mixture (mix) has been patch tested at 100 ppm (0.01%) worldwide and at 200 ppm (0.02%) in Sweden since 1986 and also in the European baseline series since 2014.

OBJECTIVE: A new aqueous mix of MCI 0.015% and MI 0.2% was compared with patch testing with the 2 aqueous baseline preparations of MCI/MI 0.02% and MI 0.2%.

METHODS: Four thousand three hundred ninety-seven patients with dermatitis in 12 International Contact Dermatitis Research Group dermatology departments from 3 continents were patch tested simultaneously with the 3 preparations.

RESULTS: The frequency of positive patch tests to the allergens varied between 0% and 26.7% in the 12 test centers. The new mixture MCI/MI 0.215% in aqua (aq) detected significantly more patients with MCI/MI allergy than both MCI/MI 0.02% aq (P < 0.001) and MI 0.2% aq (P < 0.001) alone and combined.

CONCLUSIONS: The results favor replacing the preparations MCI/MI 0.02% aq and MI 0.2% aq with the mixture MCI/MI 0.215% aq in the International Contact Dermatitis Research Group baseline series.