Contact Sensitization to Methylchloroisothiazolinone/Methylisothiazolinone is Associated With Poor Treatment Outcome: A Retrospective Study

Methylchloroisothiazolinone/methylisothiazolinone (MCI/ MI), a preservative, is commonly used in industrial chemicals and personal care products (PCPs) worldwide.The MCI/MI can induce allergic contact dermatitis (ACD), and over 20% of patch-tested North American patients exhibit sensitivity to at least one isothiazolinone 1 .MCI/MI contact allergy can trigger airborne ACD and manifest severe clinical symptoms 2 .Considering these factors, ACD induced by MCI/MI may exhibit a prolonged disease course.However, there is insufficient data regarding its therapeutic outcomes based on antigen positivity, including MCI/MI, in patients with ACD.We conducted a cluster analysis to evaluate patch test result patterns and stratify treatment outcomes of ACD.We retrospectively reviewed de-identified patch test results and relevant clinical information of patients from a single tertiary center between January 2012 and July 2022.Patch testing and interpretation were conducted according to the International Contact Dermatitis Research Group guidelines.Positive patch testing was defined as a grade of weak (+) or stronger at 96 hours.Korean standard antigens (Chemotechnique, Vellinge, Sweden) or thinlayer rapid use epicutaneous test (Mekos Laboratories, Hillerd, Denmark) were used for patch testing and a total of 19 common antigens were analyzed.Patients diagnosed with irritant contact dermatitis were excluded.To analyze patch-testing result patterns, we classified 6 clusters using the k-means clustering algorithm.A 'poor-response' was defined as the need for the administration of systemic steroids or immunomodulators, such as cyclosporin and methotrexate, for more than 30 days 3 .We compared the demographic profiles and treatment outcomes among the clusters using analysis of variance.To mitigate the risk of type I error due to multiple comparisons, we applied the Bonferroni correction for pairwise comparisons.All analyses were performed using R version 4.2.3 (R Foundation for Statistical Computing, Vienna, Austria).A total of 3,069 patch-tested patients were included in this study, of whom 1,923 (62.7%) were female.The mean follow-up duration was 463 days.Among the 6 identified clusters, the non-dominant group (n=1,862) was the most frequent, whereas the MCI/MI group (n=63) was the least frequent (Fig. 1).The average doses of systemic steroids (calculated as methylprednisolone), cyclosporin, and methotrexate were 9.05 mg/day, 120.65 mg/day, and 7.90 mg/week, respectively.The MCI/MI group showed a higher steroid-poor response rate (11.1%, n=7) than the other clusters, except for the metal-dye cluster after Bonferroni correction (Table 1).Similarly, MCI/MI group had a higher cyclosporine-response rate, although it was not statistically significant.The mean age of the MCI/MI group (38.7 years) was significantly lower than that of the fragrance and metal-dye groups in the Bonferroni correction.In the sub-analysis with a follow-up of >90 days, the MCI/ MI group consistently exhibited a higher poor steroid response