Background: This is the second United States multicenter study of human skin sensitization to methylchloroisothiazolinone/methylisothiazolinone (MCI/MI). From January 1991 through March 1992 patients treated in 11 patch test clinics were patch tested to 100 ppm MCI/MI, formaldehyde 1% aqueous, quaternium-15 2% petrolatum, imidazolidinyl urea 2% petrolatum, and paraben mix 12% petrolatum. Results: Twenty-three of the 1,259 patch test patients (1.8%) had positive patch test reactions to MCI/MI. Thirteen of the 23 (56.5%) were thought to be relevant to the presenting dermatitis. The frequency of positive patch test reactions to the other allergens was quaternium-15, 7.2%; formaldehyde, 7.1%; imidazolidinyl urea, 2.2%; and paraben mix, 1.4%. Conclusion: Skin sensitization to MCI/MI has not changed significantly since the first United States multicenter study in 1988 to 1989. The rate of positive patch test responses to MCI/MI was comparable with that for parabens and imidazolidinyl urea and significantly less than for formaldehyde and quaternium-15.
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