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Pulmonary function tests – sometimes referred to as PFTs, lung function or breathing tests – are a battery of tests that evaluate how well your lungs work. Together with patient history and other diagnostic tests, they help doctors assess respiratory symptoms, such as shortness of breath and cough, identify risks before surgery and diagnose common respiratory diseases such as chronic obstructive pulmonary disease (COPD) and asthma.
Although opinions may vary about what entails a complete set of pulmonary function tests, three of the most common are listed below:
Spirometry is the most common pulmonary function test. It requires you to perform a maneuver from a seated position with a handheld device called a spirometer. Wearing a clip on your nose, you’ll take a deep breath and exhale as hard as you can into the mouthpiece of the spirometer. It’s important you seal your lips tightly around the spirometer’s mouthpiece and rapidly exhale to the fullest extent until no further air can be exhaled. Usually, this maneuver will be performed at least three times to ensure consistent results. The highest value of the three closest test results will be used as the final result.1
Sometimes your doctor may want you to inhale a bronchodilator medication to relax and widen your airways after the initial series of tests. You’ll wait approximately 15 minutes to give the medication a chance to take effect, then do another test. Your doctor will compare the two test measurements to determine if the bronchodilator improved your lung function.
People with COPD have difficulty expelling the total amount of air they inhale. Over time, residual air left lingering in the lungs contributes to worsening lung function and breathing problems. Although spirometry is an extremely useful tool, it’s incapable of measuring certain lung volumes that are important for your doctor to be aware of. These include residual volume (the volume of air remaining in the lungs after the most forcible exhalation possible), functional residual capacity (volume of air present in the lungs at the end or a normal, passive exhalation) and total lung capacity (the volume of air contained in the lungs at the end of a maximal inhalation). Body plethysmography is a pulmonary function test that measures lung volumes such as these, that spirometry is incapable of measuring.1
During the test, you’ll be seated in an enclosed, partially see-through or totally see-though box that resembles a phone booth. The technician will ask you to wear a nose clip and will instruct you how to breathe through a mouthpiece. The test usually takes no more than 15 minutes to complete. If you use supplemental oxygen, you’ll not use it during the test. If you have difficulty with enclosed spaces, be sure to let your technician know.1
Diffusing capacity of the lungs for carbon monoxide (DLCO) is a pulmonary function test that measures how well oxygen and carbon dioxide are diffused (transferred) between the lungs and the blood stream. Diffusing capacity is often low in certain lung diseases such as emphysema, lung cancer, pulmonary fibrosis and sarcoidosis.
During this test, a mask will be placed over your face. You’ll inhale a single deep breath of gas, hold your breath for 10 seconds and then exhale. The air you exhale will be measured. The gas you inhale will contain air, carbon monoxide and trace amounts of helium. Don’t worry, the gas contains very small amounts of these elements and is not dangerous.3
When the gas is exhaled, doctors will be able to determine how much carbon monoxide and helium diffused from the alveoli (air sacs) of the lungs, across the alveolar capillaries, the smallest blood vessels traveling through the lungs, and into the blood stream. DLCO is one of the most clinically valuable tests of lung function.3
For more information about pulmonary function tests, talk to your primary health care provider or pulmonologist.
 Gildea, Thomas R. & McCarthy, Kevin. Pulmonary Function Testing. Cleveland Clinic Center for Continuing Education. August, 2010.
 Mayo Clinic. Spirometry. August 17, 2017.
 Eldridge, Lynne MD. Diffusing Capacity of the Lungs. Very Well Health.