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Background: Over the past years, it has become widely recognized that a proportion of chronic spontaneous urticaria (CSU) cases is of autoimmune origin, however, the search for reliable diagnostic tools to confirm underlying autoimmune pathophysiology is ongoing. The CD63 basophil activation test (CD63 BAT) has recently become useful for diagnosing autoimmune CSU. Objectives: Toanalyse the correlation between positive CD63 BAT results, total IgE antibody levels, and the presence of autoimmune thyroiditis as a comorbidity in patients diagnosed with CSU. Materials and Methods: We performed a retrospective analysis of CD63 BAT results obtained from 87 CSU and 20 non-CSU patients. The information extracted from the patients' records included age, gender, total IgE levels, clinical history of autoimmune thyroiditis (AT), and the presence of anti-thyroid autoantibodies. Results: Positive CD63 BAT results were significantly more frequent in CSU patients compared with non-CSU subjects ( p=0.045). Furthermore, we found a strong significant negative correlation between the stimulation index (SI) value for CD63 BAT and total IgE levels in CD63 BAT-positive CSU patients ( p=0.004; Spearman's rank correlation coefficient ρ=-0.322), meaning that higher SI values corresponded to lower total IgE values, and vice versa. Conclusion: The current standard set of diagnostic tools cannot be reliably used to determine when CSU is caused by autoimmune mechanisms. There is evidence that CD63 BAT represents a helpful diagnostic tool for detecting underlying autoimmunity. We show that high SI values in CD63 BAT-positive CSU patients correlate negatively with their total IgE levels. The clinical relevance of this effect needs to be investigated further.