FeNO test

FeNO (fractional exhaled nitric oxide) is a test to measure the levels of nitric oxide in your breath. A high level of nitric oxide when you breathe out can show that your airways are inflamed. This could be a sign that you have asthma.

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

FeNO to diagnose asthma

A FeNO test is used to help diagnose asthma.

If your doctor thinks you may have asthma, they will talk to you about your medical history to find out more about your symptoms. They may use FeNO and other tests, such as spirometry and peak flow tests, to help confirm your diagnosis.

If your symptoms are particularly bad, your healthcare professional may start your treatment before you have a FeNO test. This means you will try asthma medicines to see if they make a difference before you are diagnosed.

FeNO to monitor asthma

Your healthcare professional may use FeNO to monitor your asthma at your regular review.

It can help them know when to prescribe or change the dose of your medicine to help you stay well with asthma.

You can have a FeNO test at your GP surgery if the equipment is available. If not, you may need to go to a hospital outpatients department or a Community Diagnostic Hub.

You’ll get a letter or text from your healthcare professional telling you how to prepare for your FeNO test. Read it carefully.

Before your test, you should avoid the following as these can affect your results:

• too much physical activity or exercise for at least an hour before the test
• smoking for at least an hour before the test, if you smoke
• hot drinks, caffeine and alcohol for at least an hour before the test
• foods such as green leafy vegetables and beetroot 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. You should also leave enough time to get to your appointment, so you’re not rushed and tired before the test.

You will breathe 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 five to ten seconds to breathe into the mouthpiece.

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

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. Not all types of asthma cause high levels of nitric oxide.

Nitric oxide is measured in parts per billion (ppb). 

FeNO to diagnose asthma

Your healthcare professional will use your test results together with your medical history to help decide what type of asthma you have and how to manage it. They will explain these results to you.

• You will be diagnosed with asthma if your medical history suggests you have asthma and your FeNO result is 50 ppb or more.
• Children aged 5 to 16 will be diagnosed with asthma if their medical history suggests they have asthma and their FeNO result is 35 ppb or more.

If the FeNO test shows your airways are inflamed, this can be managed with asthma medicines like inhaled steroids. 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.

FeNO to monitor asthma

If your FeNO result is raised at your asthma review, you may have increased airway inflammation. This means you have a higher risk of having asthma symptoms or an asthma attack. Your healthcare professional will check you’re taking your asthma medicines as prescribed and your inhaler technique.

If your medicines are being taken correctly and your inhaler technique is good, your healthcare professional may decide that you need stronger treatment to reduce the inflammation. They may also refer you for specialist treatments.