The usefulness of blood eosinophil count and FeNO to predict sputum eosinophilia in the diagnosis of severe eosinophilic asthma

Background: New biologicals for the treatment of severe eosinophilic asthma have been available for several years. In order to select the suitable patients for these treatments, it is important to characterize airway inflammation properly. Aim: We investigated, if elevated blood eosinophilic count or elevated FeNO were able to predict eosinophilic airway inflammation found by examining induced sputum samples of patients with severe asthma. Methods: 45 consecutive patients suffering from severe asthma (uncontrolled asthma on GINA step 5 treatment) were examined by performing differential count of inflammatory cells in induced sputum and measuring blood eosinophilic count and FeNO. The results were then used to calculate the sensitivity and specificity and the positive and negative predictive values of elevated blood eosinophilic count (>0.3 x 109/l) and elevated FeNO (>25 ppb or >50 ppb) to predict elevated sputum eosinophilic counts (>3%). Results: We found only moderate sensitivity and specificity of both blood eosinophilic count and elevated FeNO. This leads to rather poor both positive and negative values of these measurements as predictors of airway eosinophilic inflammation. Conclusion: Blood eosinophilic count and FeNO have only a limited value in the evaluation of patients suffering from severe eosinophilic asthma.