Role of FeNO: How Can FeNO Be Positioned in Practice?

Chronic obstructive pulmonary disease (COPD) is a complex and heterogenous disease, which justifies the need for a precision medicine approach for the improvement of its assessment, treatment and outcomes. One of the treatable traits for airway diseases is “eosinophilic airway inflammation,” which can be detected by elevated fractional exhaled nitric oxide (FeNO). NO is generated by inducible NO synthase, which is transcriptionally regulated by type 2 immunity-associated cytokines interleukin (IL)-4 and 13 in airway epithelial cells of asthmatics. Recent international guidelines have employed FeNO testing to support the diagnosis of asthma in combination with respiratory symptoms and pulmonary function tests. The FeNO testing can also be utilized to detect eosinophilic airway inflammation in patients with COPD, who can respond to inhaled corticosteroid therapy. Several studies attempted to provide diagnostic criteria including FeNO. This chapter describes why and how FeNO can be used to detect type 2 airway inflammation in the clinical practice setting, which is one of the characteristics of asthmatic components in asthma-COPD overlap.