IMPORTANCE Contact allergy tomethylisothiazolinone (MI) or to the combined formulation of methylchloroisothiazolinone and MI (MCI-MI) has increased significantly, with a frequency of as much as 11.1%in patients with dermatitis; however, few cohort studies in the US population have been reported. Objective To investigate the prevalence of contact allergies toMI and MCI-MI and the outcomes of patients with positive patch test reactions to MI and MCI-MI. Design, Setting, and Participants Retrospective medical record review of all consecutive patients (n = 703) presenting with possible allergic contact dermatitis and subsequently undergoing patch testing at a tertiary referral center from January 1, 2012, through November 30, 2014. Patch testing used the North American Contact Dermatitis Group standard series. The MCI-MI concentration was 100 ppm; the MI concentration in the screening series increased from 200 to 2000 ppm in January 2013. Demographic data, exposures, and outcomes were reported for patients with positive patch test reactions to MI and/or MCI-MI. Data were analyzed from December 1, 2014, through January 31, 2015. Main Outcomes and Measures Positive patch test reaction to MI and/or MCI-MI and identification of the relevance of contact allergy to these preservatives. Follow-up after allergen avoidance was determined when available. Results Of the 703 patients who underwent patch testing, contact allergy toMI and/or MCI-MI occurred in 57 patients (8.1%), with 35 reactions to MI only, 5 reactions to MCI-MI only, and 17 reactions to both. Prevalence of contact allergy toMI increased from 6 of 236 patients (2.5%) in 2012 to 16 of 235 patients (6.8%) in 2014. The most commonly affected sites were the hands and face. Contact allergy toMI and/or MCI-MI was occupationally related in 4 cases. Cosmetics, soaps and cleansers (including wet wipes), and hair care products accounted for all identified sources. Twenty-three patients had follow-up data, and 17 of these (74%) improved with allergen avoidance. Conclusions and Relevance The increasing prevalence of contact allergy toMI alone supports its addition to the standard series to identify cases missed by testing only for MCI-MI. Recent regulations by the European Commission have banned MCI-MI in all leave-on body products as of July 16, 2015. Currently recommended US regulations are less stringent; however, US regulatory agencies must act to ensure patient safety.
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