FeNO test

Find out about the FeNO test, when it’s used, how the test is done, and what your results might mean.

What is a FeNO test?

FeNO (fractional exhaled nitric oxide) is a test that measures the levels of nitric oxide in your breath. 

A high level of nitric oxide when you breathe out can be a sign that you have inflamed airways, and could be a sign that you have asthma.

A FeNO test is suitable for adults and most children over five. 

When is a FeNO test used?

FeNO to diagnose asthma

A FeNO test is used to help diagnose asthma.

Your doctor will talk to you about your medical history, and you’ll need to have other tests too, such as spirometry and peak flow tests.

FeNO is often done alongside a trial of treatment, which is where you try asthma medicines to see if they make a difference.

FeNO to monitor asthma

FeNO is not usually used in routine appointments to monitor your asthma. But it may be used in specialist clinics to monitor some types of severe asthma if your specialist thinks it's useful.

It can help your specialist healthcare team prescribe or change the dose of your medicine to help you stay well with asthma.

You can have a FeNO test in your GP surgery if the equipment is available. Or you may need to go to a hospital out-patients department for it

How to prepare for your FeNO test

You’ll get a letter from the hospital telling you how to prepare for your FeNO test. Read it carefully.

  • Leave enough time to get to your appointment, so you’re not rushed and tired before the test.
  • You should avoid too much physical activity or exercise for at least an hour before the test.
  • If you smoke, avoid smoking at least an hour before the test.
  • Hot drinks, caffeine and alcohol can all affect the result, so avoid these at least an hour before the test.
  • Foods such as green leafy vegetables and beetroot can also affect the result, because they are rich in nitrates. Avoid eating these for at least three hours before the test.
  • You can take your regular medicines as usual before the test. But let the person doing the test know about any medicines you’ve taken.

 The NHS has guidance on how to prepare for the test.

How is the test done?

A FeNO test is done by breathing into a mouthpiece attached to a hand-held monitor.  The reading shows up on the monitor screen .
First, you need to breathe in deeply, with your mouth open.  Then you’ll be asked to breathe out slowly and steadily into the mouthpiece until your lungs are empty.

The test is very quick and easy to do.  It only takes a few seconds to breathe into the mouthpiece.

You may need to do the test a few times to get an accurate result. 

What do my results show?

The results of your FeNO test can show how inflamed your airways are.  It can help work out if you have allergic or non-allergic asthma.

This is because levels of nitric oxide in your breath are higher if you have certain types of allergic asthma.

If the FeNO test shows your airways are inflamed, this can be controlled with asthma medicines. Once you’re taking asthma medicines, your results should show lower levels of nitric oxide in your breath compared to levels before you started treatment. This is a sign the medicines are working to control the inflammation.

The NHS has developed a short leaflet to explain what FeNO is, how the test is done and how to prepare for the test.

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