Abstract Background Fractional exhaled nitric oxide (FeNO) is a valuable biomarker for eosinophilic airway inflammation, with accuracy influenced by expiratory flow rates. Objective To identify optimal FeNO flow rates for detecting airway inflammation in asthma. Methods This prospective cohort study enrolled non-smoking adult (18 healthy controls, 61 asthmatics). FeNO (45, 50, 55, and 200 mL/s), complete blood counts, sputum cytology, and spirometry were performed. Correlations between FeNOs and sputum/peripheral eosinophils were analyzed. Receiver operating characteristic (ROC) curves assessed FeNOs' predictive value for identifying sputum eosinophilia, alone or in combination with peripheral eosinophils. Results FeNO at 50 mL/s (FeNO50) exhibited the strongest correlation with peripheral eosinophils ( r = 0.51, p < 0.001) and was superior to FeNO45/55 in predicting sputum eosinophilia (AUC = 0.694 for 2% eosinophilia, 0.726 for 3% eosinophilia; p < 0.05). FeNO200 demonstrated better predictive accuracy (AUC = 0.717 at 2%, 0.743 at 3%), with cutoffs of 9.6 ppb (2%) and 10.5 ppb (3%). Combining FeNO200 with elevated peripheral eosinophil counts further improved diagnostic accuracy (AUC = 0.806 at 2%, 0.763 at 3%; peripheral eosinophil cutoffs: 155 and 275 cells/μL, respectively). Moreover, FeNOs correlated more strongly with absolute sputum eosinophil counts compared to percentages. Asthmatics exhibiting both reduced small airway indices and FEV 1 presented significantly elevated FeNO levels. Conclusion Precisely controlling expiratory flow rate at 50 mL/s improves the correlation between FeNO and eosinophil counts, while FeNO200 yields superior diagnostic performance. Combining FeNO200 with peripheral eosinophil counts offers a robust strategy for accurately detecting airway eosinophilic inflammation in asthma. Clinical Trial Registration : ChiCTR2500104677 (Date of Registration: 2025-06-20).
Wu et al. (Thu,) studied this question.