FeNO stands for Fractional exhaled Nitric Oxide— a breath test that measures nitric oxide levels in exhaled breath.
It measures the concentration of nitric oxide (NO) in exhaled breath, reported in parts per billion (ppb).
Nitric oxide is produced in the airways when there’s eosinophilic (allergic-type) inflammation, common in asthma.
The test helps to diagnose asthma, predicts response to inhaled steroids, and monitors whether asthma treatment is working.
Typical values
Below 25 ppb (adults): normal/low likelihood of eosinophilic inflammation 25–50 ppb: intermediate Above 50 ppb: high likelihood of eosinophilic inflammation
Cutoffs vary slightly for children, with generally lower thresholds.
It’s non-invasive, quick, and often used alongside spirometry to get a fuller picture of airway inflammation rather than just airflow.
Elevated levels indicate type 2 (eosinophilic) airway inflammation, which is common in many forms of asthma.
Helps support the diagnosis of asthma, especially when combined with symptoms, history, and other tests like spirometry.
Identifies patients likely to respond to inhaled corticosteroids.
Monitors airway inflammation, treatment response, adherence to therapy, and risk of exacerbations.
Assists in phenotyping severe asthma-deciding on biologic therapies.
It is not a standalone diagnostic test for asthma and does not work well for all asthma phenotypes-neutrophilic asthma often shows low FeNO.