['Measurement of sputum or blood eosinophils may allow identification of a severe eosinophilic asthma population responsive to mepolizumab.', 'The primary objective was assessment of a single blood eosinophil measurement to predict future eosinophil measurements in the following year versus using multiple blood eosinophil measurements. In addition, we examined whether a single sputum or blood eosinophil measurement was a useful biomarker for predicting treatment response to mepolizumab.', 'Based on data from placebo subjects (n = 155), we determined whether a blood eosinophil count of 150/μl or greater at screening remained on average above this level during the following year. The rate of exacerbation reduction in the sputum substudy population based on the screening blood eosinophil count and sputum eosinophils was evaluated.', 'Of 115 patients with eosinophils 150/μl or greater at screening, 98 (85%) remained above this level in their post-screening average. Using the average of two, three or four measurements 150/μl or greater, 97 (85%), 103 (90%), and 105 (92%) have postscreening averages above 150/μl. Mepolizumab reduced exacerbations by 69% (95% confidence interval [CI] = 41-83%) in subjects with baseline sputum eosinophils of 3% or greater compared with 66% (95% CI = 7-87%) in subjects with baseline sputum eosinophils under 3%. The reduction was 72% (95% CI = 41-83%) in subjects with blood eosinophils of 150/μl or greater compared with 30% (95% CI = -134 to 79%) in subjects with blood eosinophils under 150/μl.'] Is blood eosinophil count a useful biomarker to identify patients with severe eosinophilic asthma?