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An exhaled nitric oxide level test can help diagnose and manage asthma. It measures the amount of nitric oxide that is exhaled from a breath. Increased levels of nitric oxide are associated with swelling of lung airways. This test can be used to determine whether someone being treated for asthma is responding well to certain medications.

How Is It Done?

The test takes less than five minutes for a person to complete, and is similar to another breathing test called the pulmonary function test or spirometry. There will be a technician helping and guiding you.

You will be asked to:

  • Put clips on your nose.
  • Exhale, or breathe out, completely and empty your lungs.
  • Put the mouthpiece in your mouth and inhale, or breathe in slowly and deeply to fill your lungs with air.
  • Exhale slowly and steadily until you hear a beep or a light comes on. Your doctor may have you watch a computer monitor that registers how much you're breathing out so that you can maintain a steady exhalation. You will probably need to repeat the test a few times to confirm your results. 

What Do the Results Mean?

The measurements are used to determine:

  • If you should take steroids to manage your asthma.
  • If you are already taking steroids, the results will show how effective your medications are.
  • If you are following your asthma treatment plan.
  • If you should modify your therapy plan; you might be asked to do the test three to four times a year.
  • In some cases, the results are used to diagnose asthma, along with results of other diagnostic tests.  

A higher than normal level of nitric oxide means that there is inflammation (swelling) in the lining of the airways, or could mean that you have allergic asthma. Inflammation typically responds well to corticosteroid therapy.

Is This Test Safe?

This test is quick and very safe. Rarely, some people might feel a little lightheaded when they are asked to breathe out and breathe in. In this case, the technician will ask you to sit and relax for a few minutes, and if you wish, try the test again.

Page last updated: February 19, 2020

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