An exhaled nitric oxide (eNO) test can help with the diagnosis and treatment of asthma. It measures the level of nitric oxide gas in an exhaled sample of your breath. This sample is collected by having you breathe into the mouthpiece of a machine that performs the measurement.

The diagnosis of asthma is usually made using your medical history, a physical exam, and certain tests to see how well your lungs are working, such as peak flow measurement and spirometry tests. You may sometimes need tests that may trigger and then treat mild asthma symptoms. These tests are called challenge tests.

Even after these tests, the diagnosis of asthma may still be uncertain, or there may be questions about the best treatment. That's where the exhaled nitric oxide test may be helpful. Nitric oxide is produced throughout the body, including in the lungs, to fight inflammation and relax constricted muscles. High levels of exhaled nitric oxide in your breath can mean that your airways are inflamed — one sign of asthma.

Nitric oxide testing is also done to help predict whether or not steroid medications (which decrease inflammation) are likely to be helpful for your asthma. If you've already been diagnosed with asthma and treated with one of the steroid medications, your doctor may use an exhaled nitric oxide test during office visits to help determine whether your asthma is under control.

Exhaled nitric oxide testing may not be necessary or provide useful information for everyone who has asthma. In addition, it may not be available in all hospitals or doctor's offices.

Check ahead of time to see whether nitric oxide testing is covered by your insurance.

To make sure test results are accurate, you will need to avoid the following for at least two hours before you take the test:

  • Using an asthma inhaler
  • Eating and drinking
  • Exercising
  • Using tobacco, toothpaste or mouthwash

To do this test, you'll be seated. Your doctor will have you put in a mouthpiece attached with a tube that leads to an electronic measurement device. Next, you'll breathe in for two or three seconds until your lungs are filled with air. Your doctor will then have you exhale steadily so that the air flows out of your lungs at a steady rate. 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'll need to repeat the test a few times to confirm your results. The entire test generally takes five minutes or less.

Higher than normal levels of exhaled nitric oxide generally mean your airways are inflamed — a sign of asthma.

  • Levels under about 20 parts per billion in children and under about 25 parts per billion in adults are considered normal.
  • More than 35 parts per billion in children and 50 parts per billion in adults may signal airway inflammation caused by asthma.

Nitric oxide test results can vary widely from person to person. When interpreting test results, your doctor will consider a number of other factors. These may include:

  • Your asthma signs and symptoms
  • Results of other tests, such as peak flow tests or spirometry tests
  • Past nitric oxide test results
  • Medications you take
  • Whether you have a cold or the flu
  • Whether you have hay fever or other allergies
  • Whether or not you smoke
  • Your age
Mar. 19, 2014