FeNO is not affected by spirometry, IOS or walking in healthy subjects or well controlled asthma patients

Background: Measuring FeNO is a convenient, non-invasive method to assess airway inflammation. In asthmatics, elevated FeNO can predict treatment response to inhaled corticosteroids (ICS), and it can be used to monitor treatment adherence and guide step-up or step-down of ICS treatment. The aim of this study was to investigate, whether spirometry, IOS or walking affected FeNO measurement. Methods: FeNO was measured twice at baseline in 25 healthy controls and 21 patients with well-controlled asthma, and was repeated after performing IOS, spirometry and after walking 400 m. Reproducibility was assessed with a Bland-Altman test where the bias(B), upper limits of agreement (ULOA) and lower limits (LLOA) of agreement are calculated as % mean of collected data. These calculations will be written to as [B; ULOA; LLOA]. Results: At baseline mean FeNO was 16.0 (95% CI 12.3-19.8) in the healthy subjects and 20.2 (14.9-25.6) ppb in the asthmatics. There was no significant change in FeNO at baseline [0.258; 74.2;-74.8], after IOS [-9.21; 56.4;-74.9], spirometry [-3.65; 75.8;-83.1] or after walking [-4.18; 92.5;-84.1]. The limits of agreement in a Bland-Altman plot were 12 ppb, less than the clinically significant difference. There was no trend to bias in the Bland-Altman plots. Conclusion: Measurement of FeNO in healthy subjects and well-controlled asthmatics is not affected by performing IOS, spirometry, or exercise.