Fractional exhaled nitric oxide and blood eosinophils in relation to lung function, allergy and nasal polyps in asthma

Background Type 2 inflammation plays a central role in asthma, but blood eosinophil count (BEC) and fractional exhaled nitric oxide (Feno) levels may reflect different type 2 pathways. Their associations with different asthma characteristics in unselected populations remain unclear.Objectives To examine how BEC and Feno, independently and synergistically, are related to lung function, allergy, and nasal polyps in a population-based asthma cohort.Methods Data were analyzed from 1567 participants with physician-diagnosed asthma in the West Sweden Asthma Study. Lung function, bronchodilator responsiveness, and presence of allergy and nasal polyps were evaluated across categories of BEC and Feno labeled as low, medium, or high using various cutoffs. Associations were assessed using linear and logistic regression models in participants who were inhaled corticosteroid (ICS)-naive and ICS-treated separately.Results Of participants, 49% (n = 767) were on ICS treatment at the time of examination. Among those who were ICS-treated, BEC, independently of Feno, was associated with reduced FEV1 and forced vital capacity percentages, higher nasal polyps and clinical allergy, and together with Feno contributed to bronchodilator responsiveness. Interaction effects between Feno and BEC were observed primarily for FEV1 and forced vital capacity percentages, at high range of cutoff values. In contrast, among participants who were ICS-naive, neither Feno nor BEC showed independent associations with outcomes; instead, their synergistic effect was associated with lower FEV1 and forced vital capacity percentages and a higher prevalence of clinical allergy, with consistent effects across a range of cutoff values.Conclusion BEC and Feno reflect distinct yet complementary aspects of type 2 inflammation in asthma. These findings support the joint use of BEC and Feno for obstructive airway diseases phenotyping.