Isothiazolinones (MCI/MI): 200 ppm versus 100 ppm in the standard series

In an attempt to define the optimal test concentration for isothiazolinones, 200 and 100 ppm of 5-chloro-2-methyl-4-isothiazolin-3-one/2-methyl-4- isothiazolin-3-one (MCI/MI) in aq. solution were tested simultaneously in 918 consecutive eczema patients. 3.4% were positive to 200 ppm and 1.7% to 100 ppm, indicating that up to 50% of the cases might have been missed. Among the 33 cases with positive reactions, 21 showed stronger reactions to 200 than to 100 ppm, while 9 had reactions of the same strength. Use tests were carried out in 7 cases, of which 2 were positive. MCI/MI remains a peculiar allergen and some of the problematic factors are: the morphology of the test reactions, the difficulties in tracking down the exposure (past or current relevance), the multisensitivity (MCI/MI plus other allergens), the outcome of the use tests and the optimal test concentration. Our present strategy is to use 200 ppm as an indicator and then to carry out serial dilution and use tests in each "positive" case.