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Background: Current frequency and risk factors for sensitization to methylisothiazolinone (MI), methylchloroisothiazolinone/methylisothiazolinone (MCI/MI), benzisothiazolinone (BIT) and octylisothiazolinone (OIT) in Spain are not well known. Objectives: To study the frequency of sensitization, risk factors and simultaneous sensitization between the four isothiazolinones. Materials and Methods: We analysed all 2019-2021 consecutive patients patch-tested with MI (0.2% aq.), MCI/MI (0.02% aq.), BIT (0.1% pet.) and OIT (0.1% pet) within the Spanish Contact Dermatitis Registry (REIDAC). Results: A total of 2511 patients were analysed. Frequencies of sensitization were: any isothiazolinone 15.7%, MI 6.8%, MCI/MI 4.8%, BIT 3.5% and OIT 0.5%. MI and MCI/MI sensitization was associated with being occupationally active, hand dermatitis, detergents and age over 40. BIT sensitization was associated with leg dermatitis and age over 40. About one in nine MI-positive patients were positive to BIT, whereas one in five BIT-positive patients were positive to MI. Conclusions: Sensitization to MI, MCI/MI and BIT is still common in Spain, while sensitization to OIT is rare. Currently, sensitization to MI and MCI/MI seems to be occupationally related. Although its origin is unknown, sensitization to BIT is more frequent in patients aged over 40 years. Simultaneous sensitization between MI and BIT is uncommon. The Spanish Registry of Contact Dermatitis (REIDAC) is promoted by the Fundación Piel Sana (Academia Española de Dermatología y Venereología), which has received financial support from the Spanish Medicines and Health Products Agency (Agencia Española de Medicamentos y Productos Sanitarios. https://www.boe.es/boe/dias/2022/04/11/pdfs/BOE-A-2022-5975.pdf) and Sanofi. The funders were not involved in the design and conduct of the study, collection, management, analysis and interpretation of data, preparation, review, approval of the manuscript, or decision to submit the manuscript for publication.

Background: Methyldibromo glutaronitrile (MDBGN) was one of the most frequent and relevant allergens found in patch testing at the beginning of this century. In 2008, this preservative was banned from cosmetics in Europe and ever since the prevalence of contact allergy to MDBGN has progressively decreased. Despite that gradual decline, MDBGN is still patch-tested in most baseline series. This study assessed the frequency of MDBGN sensitization, epidemiological characteristics of allergic patients, and the relevance of positive patch tests in a nationwide Spanish registry (REIDAC). Patients and methods: We evaluated consecutively patch-tested patients in all participating centres. Using these data, we calculated the proportion of patients with positive patch tests to MDBGN from June 2018 to June 2020 and evaluated the relevance of the positive patch tests. Results: One hundred and fourteen out of 5072 (2.24 %) tested patients were sensitized to MDBGN. Clinical current relevance was confirmed in only one case. Conclusion: Although the frequency of contact allergy to MDBGN remains high, no clinical significance was found in most of these patients (5072 tests needed to obtain one relevant positive result). The clinical usefulness of this allergen seems weak and its continued inclusion in the European baseline series is questionable. Highlights: In 2008 methyldibromo glutaronitrile (MDBGN) was banned from cosmetics in Europe and since then, the prevalence of allergy to it has progressively decreased. Nevertheless, it is still being tested in European baseline series. This study assessed the frequency of MDBGN sensitization, epidemiological characteristics of allergic patients as well as the relevance of positive patch tests in a nationwide registry (REIDAC). The frequency of allergy to MDBGN remains high, but no clinical significance was found. Most of these patients are aged >40 years and probably represent historical sensitizations.