An exhaled nitric oxide 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.
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. This can be an indication of uncontrolled asthma, although exhaled nitric oxide may not elevated in all cases of asthma. Nitric oxide testing can be helpful in determining whether or not steroid medications, which decrease inflammation, may be helpful. 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.