Patch testing with serial dilutions of various isothiazolinones in patients hypersensitive to methylchloroisothiazolinone/methylisothiazolinone

BACKGROUND: In 2005, methylisothiazolinone (MI) on its own came into use as a preservative. Prior to that, MI was always present together with methylchloroisothiazolinone (MCI). Can the pattern of reactivity to the separate active ingredients in allergic patients tell us something about the primary sensitizer? OBJECTIVES: To investigate the potential pattern of cross-reactivity between the isothiazolinones tested, and to find the minimal elicitation concentration for each chemical, in order to determine whether the primary sensitizer is MCI or MI. METHODS: Patients reacting to MCI/MI and/or MI were additionally patch tested with MCI/MI, MCI, MI, 2-n-octyl-4-isothiazolin-3-one (OIT) and 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (dichloro-OIT) in serial dilutions. RESULTS: Three different groups of reactors were seen. One group did not react to MI; another group reacted to both MCI and MI, but had higher patch test reactivity to MCI; and a third group reacted to both MCI and MI with very similar patch test reactivity, but reacted more often to OIT and dichloro-OIT. CONCLUSIONS: Patch testing with the active ingredients of MCI/MI in serial dilutions could give information on the primary sensitizer.