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The preservative methylisothiazolinone (MI) is the American Contact Dermatitis Society Contact Allergen of the Year for 2013. Because the use of MI in cosmetics and toiletries in the United States rises, MI exposure also rises. Although it might seem likely that testing with methylchloroisothiazolinone (MCI)/MI would be adequate to pick up contact allergy to MI alone, the mix misses approximately 40% of allergy to MI, likely because of the low concentration of MI in the MCI/MI combination patch test. In Europe, several groups have documented frequency of allergy to this preservative of approximately 1.5%. The frequency of allergy to this preservative in the United States is unknown. If you are not testing for allergy to this preservative, you may be overlooking the importance of a very relevant preservative allergen that, to date, has managed to stay under the radar in the United States. This report reviews the background and reasons for adding MI to our routine screening patch testing series.

These data document the beginning of the epidemic of sensitivity to methylisothiazolinones in North America, which has been well documented in Europe. Patch testing with allergens beyond a standard screening tray is necessary for complete evaluation of occupational and nonoccupational ACD.

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