Elevated Exhaled Nitric Oxide in Adolescents Is Associated With Incident Allergic Symptoms: A Prospective Cohort Study. 
Fractional exhaled nitric oxide (FeNO) is a marker of type-2 inflammation in the airways.  Elevated FeNO may precede the development of allergic disease.  The aim of the present study was to investigate the association between elevated FeNO and the development of allergic symptoms.  A total of 959 adolescents from the general population and their parents completed a standardized questionnaire.  Lung function and FeNO were assessed at baseline.  Four years later, 921 of these individuals (96%) completed the same version of the baseline questionnaire.  Adolescents with self-reported incident allergic symptoms to cat (n=50) or dog (n=33) had higher baseline FeNO (P<.001) than those without allergic symptoms to cat and dog at both time points (n=776 and n=838, respectively).  Adolescents with incident allergic symptoms to pollen did not have elevated baseline FeNO.  The adjusted odds ratio (aOR [95%CI]) for incident allergic symptoms to cat was 4.2 (2.2-8.0) times higher if FeNO was >75th percentile (vs <75th percentile) at baseline.  This was consistent after exclusion of individuals with reported asthma, wheeze, or rhinitis at baseline (8.6 [3.0-24.1]).  Elevated FeNO in adolescents was associated with an increased risk of developing allergic symptoms to cat and dog allergens, but not to pollen allergens, after 4 years.