An exhaled nitric oxide test is one of several tests that can be used to check for asthma. It involves breathing into a mouthpiece attached to a machine that measures the level of nitric oxide gas in your breath. The body produces nitric oxide normally, but high levels in your breath can mean that your airways are inflamed (frequently a sign of asthma).
To better understand your nitric oxide test results, your doctor may ask you detailed questions about asthma signs and symptoms. You may need other tests to see how well your lungs are working, such as peak flow measurement and spirometry tests.