Fractional exhaled nitric oxide and practical points in asthma diagnosis and management.

Asthma, a multifaceted chronic inflammatory disorder, concerns more than 300 million people worldwide. Recent developments have classified asthma into predominantly Type 2 (T2) high and T2 low categories. Specific biomarkers, more so for T2 inflammation, determine the nature of inflammation. Fractional exhaled nitric oxide (FeNO) has been presented as a reliable and non-invasive biomarker of T2, eosinophilic, airway inflammation. The synthesis of nitric oxide (NO) occurs through three isoforms of nitric oxide synthase (NOS), with inducible NOS being predominantly expressed in inflammatory conditions, particularly in response to cytokines like IL-4 and IL-13. Elevated FeNO levels have been related to disease activity and have guided clinical decisions regarding corticosteroid and biologic therapies. FeNO measurement has been found to be effective in diagnosing asthma and assessing treatment response and adherence. The practicality of FeNO as a prognostic tool for exacerbation risk is highlighted in recent studies. In particular, FeNO measurements may help establish individualized treatment approaches and optimized treatment modalities, including biological treatment based on inflammation profiles. However, despite its advantages, the results of FeNO levels can be faked by many factors, such as genetics, environmental conditions, and concomitant diseases. Considering all of these, it seems helpful to integrate FeNO measurement into clinical practice in comprehensive asthma management with cautious interpretation, thus improving patient outcomes.