Older age, allergy and uncontrolled disease associate with elevated FeNO in childhood asthma

<bold>Background:</bold> Fraction of exhaled nitric oxide (FeNO) is used for diagnosis and management of asthma. However, individual factors may have an impact on FeNO, and its associations with clinical manifestations of asthma. <bold>Objectives</bold>: To investigate the association between elevated FeNO, and age, sex, allergy, airway obstruction, and uncontrolled asthma among children with asthma. <bold>Methods:</bold> This study adopted a cohort design using data from the Swedish National Airway Register. The study population consisted of 2,559 children aged 5-17 yrs, with ≥1 registration of FeNO in secondary care during 2019. Definitions: elevated FeNO, ≥25 ppb; allergy, positive to inhalant allergens; airway obstruction, FEV1 <LLN; uncontrolled asthma, Asthma Control Test score ≤19. <bold>Results:</bold> In total, 35.5% (n=908) had a FeNO value of ≥25 ppb. Children with elevated FeNO were older, and a higher proportion had allergy and uncontrolled asthma compared to the others, whereas no differences were seen regarding sex or airway obstruction. In adjusted analyses, older age (OR 1.10; 95% CI, 1.07–1.13), allergy (3.02, 2.46–3.71), and uncontrolled asthma (1.27; 95% CI, 1.05–1.55) were associated with elevated FeNO. After stratification by age (5–11 yrs and 12–17 yrs), male sex in the older children (1.32, 1.64–1.10), and uncontrolled asthma in the younger children (1.38, 1.00–1.91) was associated with elevated FeNO. In both age groups, allergy had the highest OR for elevated FeNO (5-11 yrs 3.86, 2.80–5.32; 12-17 yrs 2.53, 1.93–3.32). <bold>Conclusions:</bold> Associations between FeNO and clinical manifestations of asthma are influenced by age and sex. These findings suggest the need for age- and sex-adjusted reference values for FeNO.