Spirometry evaluates asthma by measuring how much and how fast a patient exhales, according to the University of Chicago Asthma and COPD Center. During the test, the examiner asks the patient to breathe in fully and then exhale as quickly and fully as possible into the spirometer. The patient then repeats the test after taking a common asthma medication. It the patient's lung function improves with the medicine, a diagnosis of asthma is likely.
A spirometry test requires little patient preparation, maintains the Mayo Clinic. The patient needs to wear comfortable clothing that does not hinder breathing and avoid a heavy meal right before the test. The patient sits, and a clip holds the nose closed. The doctor instructs the patient to take a deep breath and then to form a tight seal with his lips around the breathing tube while breathing out quickly and fully. The test is done three times with the highest value used as the final test result.
The ability of spirometry to accurately evaluate and diagnose asthma requires full patient cooperation and effort, states the American Academy of Family Physicians. The test is readily available at many primary health care sites but requires a trained physician to properly interpret the results. A lower test result number indicates a more severe asthma problem.