How is measuring exhaled nitric oxide useful in asthma?

Elevated fractional concentration of exhaled nitric oxide (FeNO) serves as an adjunct to history, physical examination, and spirometry testing to help with the diagnosis of asthma and is most reliable in patients who have never received steroids.
Conversely, low FeNO can help exclude asthma in the setting of normal spirometry and no suggestive symptoms.
High FeNO is an indicator of a T2-high phenotype and can predict response to inhaled corticosteroids and biologic therapies.
High FeNO is a predictor of increased exacerbation risks and accelerated decline in lung function.
FeNO can be used to monitor adherence and compliance with treatment.
Low FeNO can help to step down asthma treatment with inhaled corticosteroids.
Factors limiting the interpretation of FeNO
FeNO should not be used in isolation, and clinical history, physical examination, and spirometry testing should be incorporated.
Many factors other than asthma can raise or lower FeNO (See Table 1).
FeNO is also lower during bronchoconstriction and in the early phases of allergic response.
FeNO can be variable during viral respiratory infections.