FeNO is a measurement of nitric oxide in the exhaled breath FeNO levels are variable but levels of < 25 of parts per billion (ppb) is considered normal.
Presence of NO indicates airway inflammation. Therefore FeNo is raised in asthma and other airway inflammatory disorders.

An increase in FENO has been shown after ingestion of nitrate rich food such as lettuce and radishes as well as dietary consumption of fats and antioxidants, nitrate rich food, such as green leafy vegetables and beetroot, caffeine and alcohol can also affect the result. Smoking before the FeNO test can affect the result. Avoid smoking before the test.

FeNO is reduced following exercise and aerobic training, in patients with moderate or severe persistent asthma. It is raised in patients with sputum eosinophils and worse airway inflammation.

FeNO is a useful guide to the treatment of asthma. It helps to characterize the phenotype of severe asthma and decide on the use of different biological agents.