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DEAR MAYO CLINIC: I’ve had a cough and some wheezing that won’t go away. My doctor suspects asthma and ordered an exhaled nitric oxide test. Can you tell me more about this test?
ANSWER: Asthma is usually diagnosed based on symptoms, a physical exam and certain tests — such as peak flow measurement and spirometry tests — to see how well your lungs are working. But sometimes the diagnosis is still uncertain. To gather more clues, your doctor may use an exhaled nitric oxide test. This simple test takes only a few minutes and can be performed in your doctor’s office or in a lung function laboratory.
Asthma causes a particular inflammation of airways in your lungs. Studies have shown that an elevated exhaled nitric oxide — a gas that’s expelled when you breathe out — can be a reliable marker for asthma airway inflammation in certain patients.
Nitric oxide is produced throughout your body, including your lungs, to fight inflammation and relax constricted muscles. If your airways are inflamed, as often occurs with asthma, your body may increase its production of nitric oxide. Generally, the higher the level of exhaled nitric oxide, the greater the inflammation in the airways.
To do the test, you breathe into a mouthpiece attached with a tube to an electronic device. As you breathe out steadily, the device measures the level of nitric oxide exhaled. To make sure results are accurate, your doctor may ask you to avoid certain activities — such as eating, drinking, smoking and exercising — a few hours before the test. The test can be used to diagnose asthma, as well as to fine-tune and maintain asthma control, but your doctor will need to sort out other possible factors that can also increase nitric oxide levels unrelated to asthma.
If your nitric oxide levels are very high, your doctor may prescribe a steroid or other medications to decrease airway inflammation. Checking your levels again at a later date can help determine how treatment is working. (adapted from Mayo Clinic Health Letter) — Clayton T. Cowl, M.D., M.S., Pulmonary & Critical Care Medicine, Mayo Clinic, Rochester, Minn.