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Abstract. Objective and design:  The aim of this study was to compare the use of a late (CD63) and an early (IgE) marker of basophil activation in the flow cytometric diagnosis of beta-lactam induced allergic hypersensitivity reactions. Subjects:  Twelve patients who had had a clear cut betalactam induced immediate reaction and 16 controls were selected, as well as 11 patients who had had an immediate reaction to bee or wasp stings. Methods:  Leukocyte suspensions were incubated with allergen dilutions as well as 2 positive controls (anti-IgE and NFormyl- Methionyl-Leucyl-Phenylalanine (fMLP)). Basophils were labelled with an anti-IgE FITC (fluorescein isothiocyanate) and an anti-CD63 PE (phycoerythrin). Results were expressed as percentage CD63 expression and index calculated according to a specific algorithm including the two activation markers. Results:  Significant CD63 expression (>5 %) was observed in 3/12 cases for the beta-lactam sensitized population, in 0/16 cases for the controls and in 11/11 cases for the venom sensitized population. A significant index (determined by a ROC analysis) was observed in 11/12 beta-lactam sensitized patients and in 0/16 controls. Conclusion:  These results show that IgE (an early activation marker) is more sensitive than CD63 (a later activation marker) in the diagnosis of beta-lactam allergy. [ABSTRACT FROM AUTHOR]