Breathing and lung function tests

Exhaled nitric oxide test

What is it?

It’s a simple test to see how much nitric oxide – a substance that can be found in high amounts in people who have sensitive airways – is in your breath. It is also called a FeNO test.

What’s it used for?

If you have a breathing condition, your airways may be inflamed. The FeNO test is 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.

What happens during the test?

You breathe into a cardboard tube attached to a handheld 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 12 seconds. You may have to do the same measurement a few 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.

Reading Suggests
> 51 ppb your airways are inflamed and medication can improve your symptoms
26–50 ppb grey area – if you have symptoms, medication may help
< 25 ppb you're unlikely to have inflamed airways
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Last medically reviewed: January 2017. Due for review: January 2020

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.