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Background The diagnosis of a drug allergy is mainly based upon a very detailed history and the clinical findings. In addition, several in vitro or in vivo tests can be performed to demonstrate a sensitization to a certain drug. One of the in vitro tests is the lymphocyte transformation test (LTT), which can reveal a sensitization of T-cells by an enhanced proliferative response of peripheral blood mononuclear cells to a certain drug. Objective To evaluate the sensitivity and specificity of the LTT, 923 case histories of patients with suspected drug allergy in whom a LTT was performed were retrospectively analysed. Methods Based on the history and provocation tests, the probability (P) of a drug allergy was estimated to be >0.9, 0.5-0.9, 0.1-0.5 or <0.1, and was put in relation to a positive or negative LTT. Results Seventy-eight of 100 patients with a very likely drug allergy (P > 0.9) had a positive LTT, which indicates a sensitivity of 78%. If allergies to betalactam-antibiotics were analysed separately, the sensitivity was 74.4%. Fifteen of 102 patients where a classical drug allergy could be excluded (P <0.1), had nevertheless a positive LTT (specificity thus 85%). The majority of these cases were classified as so-called pseudo- allergic reaction to NSAIDs. Patients with a clear history and clinical findings for a cotrimoxazole-related allergy, all had a positive LTT (6/6), and in patients who reacted to drugs containing proteins, sensitization could be demonstrated as well (i.e. hen' s egg lysozyme, 7/7). In 632 of the 923 cases, skin tests were also performed (scratch and/or epicutaneous), for which we found a lower sensitivity than for the LTT (64%), while the specificity was the same (85%). Conclusion Although our data are somewhat biased by the high number of penicillin allergies and cannot be generalized to drug allergies caused by other compounds, we conclude that the LTT is a useful diagnostic test in drug allergies, able to support the diagnosis of a drug allergy and to pinpoint the relevant drug. [ABSTRACT FROM AUTHOR]