A marker of eosinophilic airway inflammation
Expansion
What it measures: nitric oxide produced by airway epithelium under the influence of type 2 inflammation, so it is a surrogate for eosinophilic airway inflammation, and it predicts corticosteroid responsiveness.
Thresholds in the diagnosis of asthma
- 40 parts per billion or more in adults: positive
- 35 ppb or more in children aged 5 to 16
- Intermediate values are interpreted alongside spirometry, reversibility and peak flow variability
Raised by: asthma and eosinophilic bronchitis, allergic rhinitis, atopy, viral infection, and a nitrate rich meal.
Lowered by: inhaled and oral corticosteroids, smoking (which reduces it markedly and can produce a false negative), bronchoconstriction at the time of the test, and spirometry performed immediately beforehand, which is why FeNO is measured before spirometry.
Clinical uses
- Supporting a diagnosis of asthma in a patient with compatible symptoms
- Assessing adherence: a persistently high value in a patient supposedly taking inhaled steroids points to non-adherence or poor technique, which are far commoner than genuinely refractory disease
- Selecting patients for biologic therapy in severe asthma, where a high FeNO alongside blood eosinophils identifies a type 2 phenotype
- Guiding steroid dose in some protocols
Limitations: it is not specific to asthma, a normal value does not exclude it, particularly in non-eosinophilic and occupational asthma, and it does not measure airflow obstruction, so it complements rather than replaces spirometry.