Discrepancies in exhaled nitric oxide results at different flow rates and their correlation with peripheral and airway eosinophils

<bold>Objective:</bold> Evaluate differences in exhaled nitric oxide (FeNO) at various flow rates in healthy individuals and asthma patients. Assess correlation between FeNO at different flow rates and peripheral blood and sputum eosinophils. <bold>Methods:</bold> Prospective study with healthy adults and asthma patients (1:3 ratio), aged 18-65. Post-bronchodilator FEV1/FVC ≥ 0.7, normal chest CT, no smoking, and no allergic rhinitis or atopic dermatitis. Three FeNO tests at 45 L/min, 50 L/min, and 55 L/min. Peripheral blood and induced sputum collected for cell counts. Record demographic and medical details. <bold>Results:</bold> A total of 91 subjects were screened, with 79 included in the analysis after excluding cases with baseline FEV1/FVC < 0.7, induction failures, or poor sputum quality. 79 subjects included (22 healthy, 57 asthma). No significant differences in age, weight, height, and gender between groups. Asthma group showed lower lung function and higher FeNO and eosinophils. Significant differences in FeNO at different flow rates within the asthma group (p < 0.001). FeNO at 45 L/min, 50 L/min, and 55 L/min were 26.10 [12.15, 66.60], 24.40 [11.20, 63.00], and 23.70 [12.50, 58.45], respectively. There was a correlation between FeNO results at different flow rates and peripheral blood eosinophils, as well as sputum eosinophils, with the strongest correlation observed at FeNO 50 (with correlation coefficients of 0.30 and 0.34, respectively). <bold>Conclusion:</bold> Precise flow rate control aids accurate assessment of airway inflammation. Flow rate detection and calibration offer potential research and translational avenues.