FENO stability in repeated measurements during three consecutive days and its association with environmental air pollution in asthma patients.

<bold>Background:</bold> Our objective was to determine the stability of fractional exhaled nitric oxide (FENO) levels in a same group of asthma patients over three consecutive days, and whether the highest levels were found in patients with higher environmental exposure. <bold>Methods:</bold> Three consecutive FENOS were determined over three days in 41 adult asthma patients (25 women and 16 men) (Visits 2-4). All determinations were made with the same device by the same qualified person. Previously they wore a personal sampler for 24 hours (visit 1) collecting their fine and coarse particulate matter (PM) personal exposure. Finally, Oxidative potential (OP) of filters was determined by two methods: dithiothreitol and ascorbic acid. <bold>Results:</bold> The correlations were very high: rho spearman>0.95 also suggesting high stability in the repeated measures ANOVA results: F(2,0; 80)=0.205, p= 0.815; Partial Eta squared= 0.005. Higher OP was not associated with greater airway inflammation, finding non-significant negative mean differences adjusted for sex, age, asthma control test, test of Adherence to Inhalers, asthma severity and body mass index. <bold>Conclusions:</bold> FENO levels were very stable, so that their repetition would not be necessary, at least within the same week, neither in clinical practice nor in epidemiological studies such as our ASTHMA-FENOP study (Santibáñez M et al. Open Respir Arch 2023;5:100246). Higher OP does not seem to be associated with greater airway inflammation. It would be interesting to compare these results with a control group without asthma and therefore without inhaled corticosteroid treatment that could slow down the possible increase in FENO.