Exhaled nitric oxide (FeNO)
What this test measures
FeNO measures nitric oxide in the breath. It rises when the airways are inflamed in the allergic pattern seen in most asthma.
Why doctors request it
UK guidance uses it when asthma is being assessed in primary care and to guide inhaled steroid treatment.
What commonly pushes it up
- Allergic (eosinophilic) airway inflammation, as in most asthma
- Hay fever season
- Eating nitrate-rich foods such as beetroot or leafy greens shortly before
- A viral cold
What commonly pushes it down
- Inhaled or oral steroids as a class
- Smoking
- Exercise shortly before the test
Everyday reasons for a value outside the range
- Diet and smoking on the day change it
- Thresholds differ between adults (25 and 40 ppb) and children
About the sample
No inhaled steroid dose change on the day; avoid caffeine, nitrate-rich food and exercise for an hour beforehand.
Upload a photo or PDF and see every line against the range your lab printed, explained in plain English. First report free.
Read my report freeThis is an educational summary of results you uploaded. It is not medical advice, a diagnosis or a treatment plan, and it does not replace the doctor who requested the test. If you are worried, or a result is marked outside range, discuss it with your doctor. Written and reviewed by a UK consultant physician. Reference ranges vary between laboratories.