Fractional exhaled nitric oxide in non-cystic fibrosis bronchiectasis: an exploratory prospective observational study

Fractional exhaled nitric oxide (FeNO) is a non-invasive biomarker of type 2 airway inflammation, but its role in non-cystic fibrosis bronchiectasis remains uncertain. This study explored the associations of FeNO with peripheral blood eosinophils, pulmonary function, high-resolution computed tomography (HRCT)-defined morphology, and short-term post-treatment change in adults with non-cystic fibrosis bronchiectasis. We conducted an exploratory prospective observational study including 152 adults with clinically and HRCT-confirmed non-cystic fibrosis bronchiectasis admitted to a national tertiary pulmonary center. FeNO was measured using a standardized single-flow technique. Pulmonary function testing, blood eosinophil assessment, and HRCT evaluation were performed. Descriptive statistics, between-group comparisons, correlation analyses, and exploratory multivariable linear regression were used. A subgroup underwent repeat FeNO assessment after antibiotic treatment. Mean FeNO was 12.1±7.9 ppb. FeNO correlated positively with blood eosinophil percentage (r=0.41, p<0.001) and MEF50% predicted (r=0.48, p<0.001) and inversely with total lung capacity % predicted (r=-0.39, p<0.001). FeNO distributions differed across ventilatory patterns (overall Kruskal-Wallis p=0.030) and HRCT morphology groups (overall Kruskal-Wallis p=0.040); the exploratory cylindrical versus cystic/saccular comparison yielded p=0.023 (unadjusted). In exploratory multivariable regression, blood eosinophil percentage remained associated with FeNO (β=1.12, 95% confidence interval: 0.29-1.95; p=0.009). Among 55 reassessed patients, FeNO decreased from 12.1±6.8 to 7.0±3.1 ppb after antibiotic treatment (p=0.010). FeNO was generally low in adults with non-cystic fibrosis bronchiectasis but showed exploratory associations with eosinophilic inflammation, selected pulmonary function parameters, HRCT morphology, and short-term post-treatment change. Larger longitudinal studies with standardized severity classification, microbiological characterization, and predefined outcomes are needed before FeNO can be recommended for routine clinical use.