Using Fractional Exhaled Nitric Oxide Measurement in Clinical Asthma Management

Asthma is a common and heterogeneous disease characterized by lower airway inflammation and airflow limitation. Critical factors in asthma management include establishing an accurate diagnosis and ensuring appropriate selection and dosage of antiinflammatory therapies. Most patients with asthma exhibit type 2 inflammation, with increased IL-4, IL-5, and IL-13 signalling, often with associated eosinophilia. Identifying lower airway eosinophilia with sputum induction improves asthma outcomes, but is time-consuming and costly. Increased type 2 inflammation leads to upregulation of nitric oxide (NO) release into the airway, with increasing fractional exhaled NO (Feno) reflecting greater type 2 inflammation. Feno can be measured easily and quickly in the clinic, offering a point-of-care surrogate measurement of the degree of lower airway inflammation. Feno testing can be used to help confirm an asthma diagnosis, to guide inhaled corticosteroid therapy, to assess adherence to treatment, and to aid selection of appropriate biologic therapy. However, Feno levels also may be influenced by a variety of intrinsic and extrinsic factors other than asthma, including nasal polyposis and cigarette smoking, and must be interpreted in the broader clinical context, rather than viewed in isolation. This review discusses the clinical application of Feno measurement in asthma care, from diagnosis to treatment selection, and describes its place in current international expert guidelines.