Background: The aspirin provocation test, considered to be the gold standard in the diagnosis of aspirin sensitivity, may be associated with severe adverse reactions; thus, alternative procedures with a higher safety profile are highly desirable. Although the cellular antigen stimulation test (CAST) has been proposed to be such an alternative, there is limited information about its clinical usefulness. Objective: It was the aim of our study to evaluate the clinical usefulness of CAST in the diagnosis of aspirin sensitivity. Material and Methods: Patients with aspirin-sensitive asthma and/or nasal polyps (n = 40), patients with aspirin-tolerant asthma and/or nasal polyps (n = 13) and healthy volunteers (n = 26) were included. A 2-day, single-blind placebo-controlled oral aspirin provocation test was performed. In vitro release of cysteinyl leukotrienes (Cys-LTs) by peripheral blood leukocytes was measured after stimulation with both stimulation buffer and lysine aspirin (2.5 mg/ml) by CAST. Results: Baseline Cys-LT levels were similar among the 3 groups. After lysine aspirin stimulation, the net increase in Cys-LTs was significantly higher in patients with aspirin sensitivity (median 91 pg/ml, interquartile range 22-206) compared with aspirin-tolerant patients (20 pg/ml, range 0-46) and healthy controls (23 pg/ml, range 0-71; p = 0.004). The assay had a sensitivity of 25%, a specificity of 92.3%, and positive and negative predictive values of 28.7 and 90.7%, respectively. Conclusion: Although the leukocytes of patients with aspirin sensitivity produce higher amounts of Cys-LTs as measured by CAST, the low sensitivity and predictive values limit the clinical usefulness of this test in the diagnosis of aspirin sensitivity. Copyright © 2008 S. Karger AG, Basel.