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FeNO test (exhaled nitric oxide)
Category: Lung & Breathing Tests · All lung & breathing tests
What it is
The FeNO test measures nitric oxide in your breath, a marker of a particular type of airway inflammation (eosinophilic/Th2 inflammation) that is typical of allergic asthma and predicts response to inhaled steroids.
Why it is done
It supports asthma diagnosis when spirometry is inconclusive, helps predict and monitor response to inhaled corticosteroids, and can flag poor inhaler adherence or ongoing allergen-driven inflammation in asthma reviews.
How to prepare
Avoid eating nitrate-rich foods (leafy greens, beetroot), caffeine and smoking for a few hours before, as these distort the reading; your appointment letter will specify. Tell the team about current inhaler and steroid use and recent respiratory infections.
What happens during the procedure
You inhale fully through the device then exhale slowly and steadily for about 10 seconds against a slight resistance, guided by an on-screen animation. It may take a few attempts to get a valid reading and is painless. The result is available immediately.
Afterwards — recovery & results
You can resume normal activities straight away. The number is interpreted alongside your symptoms, spirometry and history in clinic — it is one piece of the asthma picture, not a stand-alone verdict.
Risks & considerations
There are no risks. Readings are lowered by smoking and steroid inhalers and raised by hay fever and recent allergen exposure, so context matters.
Watch: FeNO test (exhaled nitric oxide) videos
Real procedure videos from the MedicalVideos.com library. Some clinical footage is graphic — that is the nature of a medical video library.
Related guides on MedicalVideos.com
Frequently asked questions
My FeNO is high but I feel fine — why?
Inflammation can be active before symptoms are obvious, or driven by allergen exposure. Your clinician will weigh it with the whole picture rather than treat a number in isolation.
Does a normal FeNO rule out asthma?
No — some asthma types have low FeNO, and steroids suppress it. A normal result makes steroid-responsive asthma less likely but does not exclude asthma.
Related procedures — Lung & Breathing Tests
Browse the F section of the A–Z list or return to the Procedures & Tests hub.