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Breathing and lung function tests

Fractional exhaled nitric oxide (FeNO) test

What is nitric oxide (NO)?

Nitric oxide (NO) is found in the atmosphere, and is higher in areas with air pollution. It is also produced in the body, especially if there is inflammation. A higher level of nitric oxide measured in your breath may be a sign of asthma.

What is a fractional exhaled nitric oxide (FeNO) test?

A fractional exhaled nitric oxide test is a simple test to see how much nitric oxide is in your breath. It’s often called a FeNO (“fee-no”) test for short.

What is the FeNO test used for?

If you have a breathing condition, your airways may be inflamed. The FeNO test can be useful to help your doctor understand if you have asthma. The measurements may also help your health care professional check if the medication you are taking is working.

Only certain types of asthma may be identified with a FeNO test. Your doctor will talk to you about your clinical history, and you’ll need to have other tests too, such as spirometry and a peak flow test.

How should I prepare?

Smoking before the FeNO test can affect the result, so avoid smoking beforehand if you can.

Nitrate rich food, such as green leafy vegetables and beetroot, caffeine and alcohol can also affect the result, so do not eat or drink these for an hour or so before the test.

What happens during a FeNO test?

You breathe into a plastic mouthpiece or a cardboard tube attached to a monitor. The monitor shows the reading on its screen.

You will breathe in deeply, with your mouth open, and then breathe out little by little until your lungs are empty. The breath out will normally take 10 seconds in adults (6 seconds in children). Some devices will make a sound to help keep the flow at the right level throughout the test.

 You may have to do the same measurement up to 3 times.

What will the results look like?

The results will be in parts per billion (ppb) of nitric oxide in your breath. Your health care professional will also check your symptoms, such as cough, wheeze and shortness of breath.

More than 40ppb of nitric oxide indicates that your airways are inflamed, and it’s likely that you have asthma.

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Last medically reviewed: January 2020. Due for review: January 2023

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.