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Background Peripheral blood basophils are reduced in some chronic urticaria patients when counted with granule stains. Approximately 30% of patients with severe chronic urticaria have functional autoantibodies which release histamine from healthy donor basophils in vitro but the relationship between basophil numbers in vivo and serum histamine releasing activity has not been studied. Objective To determine the relationship between basophil numbers and serum basophil histamine releasing activity and to assess whether basophils are present, but undetectable, in peripheral blood with granule stains by using a new Row cytometric method based on surface immunophenotype. Methods Basophils were counted manually by a chamber method using a granule stain and by flow cytometry using dual staining with anti-IgE and anti-Fc ∈ RI in 25 chronic idiopathic urticaria patients and 25 healthy controls. Serum histamine releasing activity was assessed on healthy donor basophils in vitro and by the weal response to autologous serum skin testing in vivo (patients only). Results Basophils were significantly reduced in chronic urticaria by manual counting and how cytometry. A subgroup of seven patients with in vitro histamine releasing activity showed a marked reduction or absence of basophils by both methods. There were no obvious distinguishing clinical characteristics between these patients and the others; six of them showed positive autologous serum skin-test responses. On comparing the two methods, the manual basophil counts were generally lower than flow cytometric counts. Agreement over the full range of values was not strong and therefore counts obtained by the two methods are not directly interchangeable. Conclusion Basopenia in chronic idiopathic urticaria is associated with serum basophil bistamine releasing activity in a subgroup of patients. The lack of granule and surface immunophenotype staining suggests a reduction in numbers rather than an inability to detect circulating degranulated cells by conventional counting methods. [ABSTRACT FROM AUTHOR]