Prevalence of Contact Sensitisation Among Patients With Atopic Dermatitis in Thailand: A 20‐Year Retrospective Study

The association between atopic dermatitis (AD) and contact sensitisation remains unclear, and data from Asian populations are limited [1]. This study aimed to assess the prevalence, associations and evolving trends in patch test reactions among individuals with AD referred for patch testing. A retrospective study approved by the Siriraj Institutional Review Board (COA no. Si 823/2022) included consecutive patients who underwent patch testing at the Contact Dermatitis Clinic, Siriraj Hospital, between 2002 and 2021. AD was diagnosed by dermatologists; other patients were classified as non-AD. Patch testing was performed using the Siriraj baseline series, adapted from the European and International baseline series, with additional series when clinically indicated [2]. Testing and readings followed International Contact Dermatitis Research Group guidelines [3, 4]. Categorical variables were compared using chi-square or Fisher exact tests, and binary logistic regression was used to estimate odds ratios. A p value < 0.05 was considered significant. Of 5917 patients, 1401 (23.7%) were diagnosed with AD. Demographic characteristics are shown in Table S1. The proportion of patients with AD increased over time, accounting for 12.7%, 14.3%, 12.7%, and 39.9% in 2002–2006, 2007–2011, 2012–2016 and 2017–2021, respectively. Patch test positivity declined over time in both AD and non-AD groups. A higher positivity rate was observed in patients with AD in 2002–2006 (91.4% vs. 79.5%, p < 0.05), with no significant differences in subsequent periods (2007–2011: 83.7% vs. 81.6%; 2012–2016: 68.0% vs. 70.6%; 2017–2021: 63.5% vs. 60.7%; all p > 0.05) (Figure S1). Over the 20-years period, positive reactions to at least one allergen were significantly lower in patients with AD than in non-AD patients (67.9% vs. 70.7%, p = 0.042) (Table 1). Patients with AD showed higher reaction rates to preservatives but lower rates to medications and rubber allergens. Methylisothiazolinone, methylchloroisothiazolinone/methylisothiazolinone and hydroperoxides of linalool were significantly more frequent in patients with AD, whereas fragrance mix I was less frequent. This large, long-term retrospective study of Thai patients undergoing patch testing shows that although the proportion of patients with AD increased over time, contact sensitisation declined in both AD and non-AD groups. This trend may reflect increased awareness and earlier screening of patients with AD, as well as changes in environmental exposure to emerging sensitisers. Notably, several allergens among the top 10 were not included in the standard baseline series, suggesting that evidence-based refinement of patch test series may be necessary. A tailored allergen panel focusing on allergens with a prevalence greater than 1% in patients with AD may therefore be useful for clinical practice and epidemiologic comparison (Table 2). The association between AD and contact sensitisation remains controversial. In our study, contact sensitisation was slightly less prevalent in patients with AD than in non-AD patients. Although a 2017 systematic review and meta-analysis reported no overall association, subgroup analyses demonstrated an inverse relationship in patch-tested populations [1]. The Th2-skewed inflammation in AD may suppress Th1-mediated delayed hypersensitivity responses, and patients with AD may exhibit higher elicitation thresholds and reduced circulating T lymphocytes [5]. This reduction in sensitisation is particularly evident in severe AD [6]. This may partly explain the lower positivity rates observed in our referral-based center. Notably, half of the top 10 allergens in our study overlapped with those reported in a previous study in Singapore, highlighting regional similarities, whereas geographic differences in exposure likely contribute to variations in allergen profiles [7, 8]. These findings suggest that a tailored allergen panel for patients with AD, focusing on frequently encountered preservatives and fragrances, may be useful for clinical practice and epidemiologic comparison. However, this study is limited by its single-center, retrospective design, selective allergen testing, and inclusion of patients referred for patch testing, which may overestimate sensitisation rates compared with the general population. In conclusion, although patients with AD showed fewer positive patch test reactions than non-AD patients, overall sensitisation remained common, supporting the use of patch testing in patients with AD with suspected concomitant contact dermatitis. Silada Kanokrungsee: conceptualization, methodology, writing – original draft, writing – review and editing, project administration, investigation. Rungsima Kiratiwongwan: writing – original draft, project administration, formal analysis. Nicha Wareesawetsuwan: data curation, formal analysis. Sasitorn Yenyuwadee: data curation, formal analysis. Titinun Kumpangsin: formal analysis, project administration, data curation. Waranya Boonchai: resources, supervision, writing – review and editing, conceptualization, methodology. Pichanee Chaweekulrat: conceptualization, supervision, methodology. The authors have nothing to report. The authors declare no conflicts of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions. Figure S1: The prevalence of atopic dermatitis in patients underwent patch tests and the percentage of at least one positive patch test reaction in AD and non-AD patients during the 20 years. Abbreviations: AD, atopic dermatitis. *p < 0.05. Table S1: Demographic data of the patients. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.