Exhaled Nitric Oxide (eNO) Testing
A nitric oxide (eNO) test assists in the diagnosis of asthma. During the examination, the patient breathes into a mouthpiece attached to a machine that measures the amount of nitric oxide in the patient's breath. While there are several breathing tests, called challenge tests, typically used to diagnose asthma, at times their results are inconclusive. It is in such cases that the nitric oxide test is helpful. The test is based on the fact that nitric oxide is produced in the lungs and elsewhere in the body, for the purpose of fighting inflammation and relaxing constricted muscles. For this reason, high levels of exhaled nitric oxide indicate that the airways are inflamed, a sign of asthma.
Reasons for a Nitric Oxide Test
There are several reasons that a nitric oxide test may be administered. If, after a comprehensive physical examination, a full medical history, and challenge tests such as peak flow measurement and spirometry, the diagnosis is still not clear, the doctor may turn to a nitric oxide test. Other reasons to administer nitric oxide testing include assessing whether steroid medications will be helpful in decreasing inflammation, or evaluating whether steroid medications that have already been administered, are keeping inflammation under control.
Not all asthma patients undergo nitric oxide tests, and the machinery necessary for these tests is not always available in every doctor's office or hospital.
The Nitric Oxide Test Procedure
In order to ensure accurate results, patients are instructed to avoid the following for at least 2 hours prior to taking the test:
- Using an asthma inhaler
- Eating or drinking
- Using tobacco
- Using toothpaste or mouthwash
During this test, the patient is seated and instructed to insert a mouthpiece attached to an electronic measuring device. For 2 or 3 seconds, the patient inhales, filling the lungs with air, then exhales steadily. Because it is important that the exhalation be steady, the patient watches a computer monitor that gives immediate feedback to ensure this occurs. The test is repeated a few times to confirm accuracy. In total, the nitric oxide test takes approximately 5 minutes.
Nitric Oxide Test Results
Abnormally high levels of exhaled nitric oxide usually mean that the patient's airways are inflamed, one indication of asthma. The doctor has to consider of variety of factors in interpreting the results of this test, including:
- Other symptoms of asthma
- Results of other breathing tests
- Results of past nitric oxide tests
- Medications the patient takes
- Whether the patient has a cold or other respiratory infection
- The patient's age
- Whether the patient is a smoker
- Whether the patient has allergies
Normal results of a nitric oxide test are 20 parts per billion for children and 25 parts per billion for adults. Levels higher than 35 parts per billion in children and 50 parts per billion in adults signify the inflammation of airway passages and indicate the presence of asthma.
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