Diagnosis of drug hypersensitivity is indeed based on history and clinical manifestations and on skin and drug provocation tests. There are limitations to the interpretation of in vivo tests as co-factors at the time of onset, the metabolism of drug, the presentation of haptens to the immune system may be different after epicutaneous or oral challenge. Therefore, biological tests would be a highly desirable solution for establishing the nature of the culprit agent. In the present work, the results of the CD63 expression tests on basophils were analysed using a novel distribution depending on the time delay between the reaction and the drug intake, the tested drug concentration and the chemical drug structure, in both immediate and non-immediate reactions. The importance of cut-off values was assessed in immediate and non-immediate reactions induced by “haptenic” molecules. The discordance between the results of specific IgE to ?-lactams and the CD63 expression test on basophils was analysed in ?-lactam allergic patients. The subgroup of patients with severe drug-induced reactions was assessed to evaluate the results of CD63 expression tests in clinical practice.