Exhaled nitric oxide fraction (FENO) values can be easily measured using portable analysers and are a surrogate marker of airway eosinophilia. FENO may be useful in diagnosing and monitoring conditions characterised by airway eosinophilia, i.e. asthma and possibly COPD. Many factors other than asthma and COPD affect FENO, especially atopy, which is associated with elevated FENO. One guideline recommends that FENO should be used as part of the diagnostic pathway for asthma diagnosis in adults and children aged >5 years. The role of FENO in monitoring asthma is even less clear, and most guidelines do not recommend its use outside of specialist asthma clinics. Currently, FENO is not recommended for diagnosis or monitoring of COPD. Although FENO is starting to find a place in the management of asthma in children and adults, considerably more research is required before the potential of FENO as an objective measurement in asthma and COPD can be realised.Key pointsFor individuals aged ≥12 years, FENO is not recommended by all guidelines as a test to diagnose asthma (recommended only by the UK National Institute for Health and Care Excellence guideline for asthma symptoms, which are likely to respond to corticosteroid treatment).FENO may be used in conjunction with other investigations to diagnose asthma in 5–16-year-olds where there is diagnostic uncertainty, but further evidence is required.FENO is not recommended as a routine test to monitor all patients with asthma or to titrate asthma treatment.FENO is not recommended for routine clinical testing in adults with COPD.FENO may be useful to identify patients with COPD who could benefit from the use of inhaled corticosteroids (asthma–COPD overlap).Educational aimsTo understand what factors other than asthma and COPD affect FENOTo understand the current controversies in the application of FENO to diagnosis and management of asthma in childrenTo understand the current controversies in the application of FENO to diagnosis and management of asthma and COPD in adults
Values of fractional exhaled nitric oxide for cough-variant asthma in children with chronic cough
