Just as people with high blood pressure or diabetes need measurements of blood pressure and blood glucose, respectively, people with lung diseases need tests of their pulmonary function. Pulmonary function tests measure lung capacity and reveal patterns characteristic of particular lung diseases. A person’s measurements are compared with those expected for a healthy person of the same age, height, and gender.
Commonly performed pulmonary function tests include spirometry, lung volume tests, and diffusion capacity tests. Spirometry measures the volume of air that can be exhaled and the rate of exhalation. The two most important values that spirometry measures are forced expiratory volume in one second (FEV1) and forced vital capacity (FVC). FEV1 measures the amount of air expelled from the lungs in the first second of a forced exhalation, while FVC is a measure of the total volume of air exhaled. These values are calculated automatically by the spirometer, along with the ratio comparing the amount of air exhaled in the first second with the total amount of air exhaled: FEV1/FVC.
Lung volume tests measure the amount of air in the lungs at different levels of inflation, and diffusion capacity testing measures how well gas moves across the membranes in the lungs. These tests can help characterize the lung abnormality as primarily obstructive (in which the airways are narrowed), primarily restrictive (in which the ability of the lungs to expand is impaired), or a combination of the two. They also determine whether the deficit in lung function is mild, moderate, or severe. People with suspected asthma may need to be tested before and after inhaling medication to see if lung function is improved.
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