Diagnosing persistent blood eosinophilia in asthma with single blood eosinophil or exhaled nitric oxide level

Background: Eosinophilic asthma is characterized by persistently elevated blood eosinophils, adult-onset asthma and corticosteroid resistance. For stratified medicine purposes one single measurement of blood eosinophils or exhaled nitric oxide (FeNO) is commonly used. The aim of this study was to investigate in patients with new-onset asthma whether persistent blood eosinophilia can be predicted with one single measurement of these biomarkers. Methods: Blood eosinophils and exhaled nitric oxide levels were measured at yearly intervals over 5 years in 114 adults with new-onset asthma on inhaled corticosteroid treatment. Two definitions of persistent blood eosinophilia were used (1); blood eosinophils at every visit ≥0.30 × 109/L, or (2) ≥0.40 × 109/L. Receiver operating characteristic analyses were performed. Diagnostic cut-off values were defined at a positive predictive value of 95% (or the highest achievable). Results: Using definition 1 (blood eosinophils ≥0.30 × 109/L) the cut-off value for a single measurement of blood eosinophils was 0.47 × 109/L. For definition 2 (≥0.40 × 109/L) the cut-off value was 0.49 × 109/L. Cut-off values for persistently low blood eosinophils were 0.17 × 109/L for definition (1) and 0.21 × 109/L for definition (2), respectively. For FeNO no cut-off values with sufficient accuracy could be defined. Conclusion: We showed that by using high and low cut-off values, one single measurement of blood eosinophils, but not FeNO in the initial phase of new-onset asthma in adults can be used to predict persistence or absence of blood eosinophilia in asthma.