Serum concentrations of total immunoglobulin E (IgE) and free IgE were measured throughout the 24-week blinded treatment period, as target engagement biomarkers of omalizumab, using validated quantitative immunoassays with lower limits of quantification of 2 and 0.83 International Units per millilitre (IU/mL), respectively, and upper limits of quantification (ULQ) of 5000 and 62.5 IU/mL, respectively. The free IgE assay had limited range to measure circulating levels of free IgE in the presence of complexes of omalizumab-IgE. According to the analysis plan for the free IgE assay, results above ULQ were set to 62.5 IU/mL. If results for one-third or fewer of the participants were greater than the ULQ, then all summary statistics were to be reported. However, if the results for more than one-third of participants were greater than the ULQ, then only the median, interquartile range and minimum were calculated, and the mean, standard deviation, and maximum were non-reportable.