FeNO is a measurement of nitric oxide in the exhaled breath. FeNO levels are variable, but levels of <25 parts per billion (ppb) are considered normal.
The 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 foods such as lettuce and radishes, as well as dietary consumption of fats and antioxidants. Nitrate-rich foods such as green leafy vegetables and beetroot, caffeine, and alcohol can also affect the result. Smoking before the FeNO test can affect the results. 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 for the treatment of asthma. It helps to characterize the phenotype of severe asthma and decide on the use of different biological agents