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.