[The role of fractional nitric oxide in exhaled breath (FeNO) in clinical practice of asthma].

The measurement of FeNO (fractional nitric oxide concentration in exhaled breath) is a noninvasive method to assess airway inflammation. The elevated level of FeNO in asthma patients was first reported in 1993, and measurement has been covered by insurance in Japan since April 2013. NO is generated through the conversion of L-arginine to L-citrulline by the action of nitric oxide synthase (NOS) and iNOS is highly expressed in asthmatic airways. FeNO is expiratory flow-ependent, and measurement at 50 mL/s is recommended. As a nitrate-rich diet and the contamination of nasal NO increase, and smoking and spirometry decrease FeNO, these factors should be avoided or taken into account when measuring FeNO. For clinical application, FeNO may be used for the diagnosis or as a guide to treat asthma. Although FeNO correlates well with eosinophilic airway inflammation, it is not present in all asthma patients, and eosinophilic bronchitis or a part of COPD also shows eosinophilic inflammation. Thus, FeNO may be used as an adjunct for asthma diagnosis, and in ATS guideline, it is recommended to use FeNO for the diagnosis of eosinophilic airway inflammation and determining the likelihood of responsiveness to steroids. Several studies attempted to use FeNO to determine the dose of ICS (inhaled corticosteroids) and compared FeNO with traditional guideline-based management. The results were inconsistent, and the Cochrane review found that FeNO-based treatment did not lead to a reduction in the ICS dose or improved asthma outcomes. Nevertheless, a study of a pregnant woman showed reduced exacerbations with a refined algorithm, and further research will shed light on the appropriate application of FeNO measurement for asthma management. In this paper, the mechanism of NO generation, background and method of FeNO measurement, and clinical application will be reviewed and an outline of the official ATS clinical practice guideline will be introduced.