FeNO

FeNO, or fractional exhaled nitric oxide, is a noninvasive biomarker of airway inflammation, produced mainly by the airway epithelium. Its level in exhaled breath increases with eosinophilic, Th2-type inflammation and often decreases when inhaled corticosteroid therapy reduces airway inflammation. FeNO measurements provide a snapshot of ongoing inflammatory activity in the airways and are used to support asthma management in addition to clinical assessment and lung function testing.

Measurement of FeNO involves collecting a bubble-free exhaled breath at a controlled flow rate, typically 50 milliliters per second. The exhaled nitric oxide is analyzed by chemiluminescence or electrochemical sensors and reported as a concentration in parts per billion (ppb). Standardized procedures require a steady exhalation, appropriate mouthpiece use, and avoidance of substances that can contaminate samples, such as recent smoking or environmental NO.

In clinical use, FeNO helps identify eosinophilic airway inflammation and can guide decisions about inhaled corticosteroid therapy, including initiation, adjustment, and titration, as well as assessing adherence. It is commonly used as an adjunct to symptom assessment and spirometry rather than as a stand-alone diagnostic test for asthma.

Interpretation of FeNO values varies by age and device, but general thresholds are commonly cited as follows: in adults, normal is roughly ≤25 ppb, borderline to elevated around 25–50 ppb, and high above 50 ppb; in children, normal is roughly ≤20 ppb, with elevations typically above about 35 ppb. Values must be interpreted in the clinical context, as FeNO can be influenced by atopy, recent smoking, infections, and corticosteroid use, and it is not specific to asthma alone.