Correlation between exhaled nitric oxide, sputum eosinophils, and methacholine responsiveness in patients with mild asthma

Background - Eosinophils in induced sputum and exhaled nitric oxide (NO) are currently used as non-invasive markers in the assessment of airway inflammation in asthma. As both sputum eosinophils (%) and exhaled NO are raised in asthmatic subjects not receiving inhaled steroids and decreased following corticosteroid therapy, a relationship between them is plausible. Methods - Exhaled NO was measured by chemiluminescence analyser, sputum induction by 3.5% saline inhalation, and bronchial responsiveness was measured as PC20FEV1 methacholine in 35 stable asthmatic patients using beta(2) agonist alone and the correlation between these non-invasive markers of airway inflammation was studied. Results - There were significant correlations between exhaled NO and PC20 (r = -0.64), exhaled NO and sputum eosinophils (%) (r = 0.48), and also between sputum eosinophils (%) and PC20 (r = -0.40). Conclusion - The correlation between exhaled NO and PC20 suggests that exhaled NO or the mechanisms leading to its increase may contribute to airway hyperresponsiveness in asthma. Furthermore, the relationship between sputum eosinophils (%), exhaled NO, and PC20 highlight the potential use of eosinophils (%) in induced sputum and exhaled NO to monitor the severity of asthma.