Please ensure Javascript is enabled for purposes of website accessibility
We Are Closed

Fractional exhaled nitric oxide (FeNO)

What is it?

Fractional exhaled nitric oxide (FeNO) is a test used to diagnose asthma by measuring your breath. The test is simple and easy to do and, alongside other diagnostic tests, can tell you and your asthma team what type of asthma you have.

Why is it important?

FeNO is useful to help you build a better understanding of your asthma and to manage it with the
support of your asthma team. The FeNO test measures inflammation in your airways. If the inflammation
is not treated it can cause life threatening asthma attacks.

By knowing what kind of asthma you have, you can:

  • Understand your medication better
  • Recognise when your medicines are working well
  • Avoid life threatening asthma attacks.


How the test is done

The FeNO test is safe and easy to do for adults and children.
You will take a big breath in and then gently breath out through the filtered mouthpiece into a small portable device for approximately 10 seconds. The test is much easier to do than other asthma tests such as spirometry or peak flow. The device shows the reading on its screen.

How to prepare for the test

Ahead of the test, there are a few things that you should try to avoid to ensure that the test is effective:

  • Leave in plenty of time and avoid exerting yourself at least 1 hour prior to your appointment
  • Avoid smoking at least 1 hour before your test
  • Hot drinks, caffeine and alcohol should be avoided for at least 1 hour before your test
  • Avoid nitrate rich foods such as green leafy vegetables (i.e. celery, leek, beetroot, lettuce and spinach) at least 3 hours before test.

You can take your regular medications as normal before the test. Ensure that the clinician is aware of any medication that has been taken beforehand or if any of the activities above have taken place prior to the test.



For more information:

This information is also available in different languages and as audio here: NHS Accelerated Access Collaborative » Fractional exhaled nitric oxide (FeNO) patient leaflet (


Information sourced from Wessex AHSN toolkit with thanks