Exhaled nitric oxide measurements: clinical application and interpretation

The use of exhaled nitric oxide measurements (FENO) in clinical practice is now coming of age. There are a number of theoretical and practical factors which have brought this about. Firstly, FENO is a good surrogate marker for eosinophilic airway inflammation. High FENO levels may be used to distinguish eosinophilic from non-eosinophilic pathologies. This information complements conventional pulmonary function testing in the assessment of patients with non-specific respiratory symptoms. Secondly, eosinophilic airway inflammation is steroid responsive. There are now sufficient data to justify the claim that FENO measurements may be used successfully to identify and monitor steroid response as well as steroid requirements in the diagnosis and management of airways disease. FENO measurements are also helpful in identifying patients who do/do not require ongoing treatment with inhaled steroids. Thirdly, portable nitric oxide analysers are now available, making routine testing a practical possibility. However, a number of issues still need to be resolved, including the diagnostic role of FENO in preschool children and the use of reference values versus individual FENO profiles in managing patients with difficult or severe asthma.