An exhaled nitric oxide test can help with the diagnosis and treatment of asthma. Also known as the fractional exhaled nitric oxide (FeNO) test, this test measures the level of nitric oxide gas in an exhaled sample of your breath. This sample is collected by having you breathe slowly and steadily into a mouthpiece that's attached to 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 need tests that trigger and then treat mild asthma symptoms. These tests are called challenge tests.

If the diagnosis of asthma is still uncertain, or if there are questions about the best treatment, an exhaled nitric oxide test may be helpful. Nitric oxide is produced throughout the body, including in the lungs, to fight inflammation and relax tight 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.

To make sure test results are accurate, avoid the following for at least an hour before you take the test:

• Smoking
• Caffeine
• Alcohol
• Nitrate-rich foods, such as leafy green vegetables and beets

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

To do this test, you'll be seated. Your doctor will have you put in a mouthpiece that's attached to a tube that leads to an electronic measurement device. Next, you'll breathe in slowly and deeply until your lungs are filled with air. Your doctor will then have you exhale slowly and steadily so that the air flows out of your lungs at a steady, even 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.

An oral exhaled nitric oxide value more than 40 parts per billion for adults and more than 25 parts per billion for children and adolescents is considered elevated.

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
• Past nitric oxide test results
• Results of other tests, such as peak flow tests or spirometry tests
• 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