A novel sandwich ELISA for the quantitative and sensitive determination of IL-13 in human serum and plasma was established. The assay employs an incubation step at acidic pH, which was shown to decrease nonspecific binding and interference from IL-13 binding proteins. The assay was validated and was shown to be accurate and precise over the entire quantification range (0.59 to 68.4？pg/mL in human EDTA plasma). The validated assay was successfully applied to samples from healthy volunteers and patients with atopic seasonal rhinitis. The assay is suitable for use in clinical trials to monitor efficacy or pharmacodynamic effects of drug candidates. 1. Introduction Interleukin-13 (IL-13) is an immunoregulatory cytokine secreted by T helper 2 lymphocytes. As being involved in the activation of IgE production, eosinophil infiltration, mucus hyper secretion, and subepithelial fibrosis, IL-13 is a central mediator in the inflammation of airways and in the pathogenesis of asthma and allergy [1, 2]. This cytokine and its receptors have therefore emerged as important targets and biomarkers for new therapeutic approaches to the treatment of asthmatic and allergic diseases [1]. In human, IL-13 has already been measured in bronchoalveolar lavage fluid (BAL) allowing to distinguish asthmatic children from control subjects [3]. However, BAL samples are very difficult to obtain, and therefore studies in children have been restricted [4]. IL-13 has also been measured in other airway fluids such as nasal lavage fluid [5], nasopharyngeal aspirates [6], and sputum [7], but all these matrices also require inconvenient collection procedures. There have been several attempts to measure IL-13 in less invasive fluids such as serum, using different immunoassay methods. These studies report a broad range of IL-13 concentrations in the serum from healthy subjects: from 0.25？pg/mL using a microparticle-based immunoassay [8, 9] to 8.1 and 92.3？pg/mL using two different commercial ELISA methods [10, 11]. While the two studies using ELISA methods could correlate systemic IL-13 concentration with asthmatic status, the third study could not distinguish healthy and asthmatic subjects based on this measurement. It therefore appears that the determination of IL-13 concentrations in serum is method dependent, and this may reflect different method performances. Particular care should be taken to validate the method in terms of specificity, sensitivity, and reproducibility to qualify its performances for the accurate measurement of endogenous IL-13 in human serum. We also hypothesize that
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