What are pulmonary function tests?

Pulmonary function tests (PFTs) measure how well your lungs take in and exhale air and how efficiently they transfer oxygen into the blood. There are several different tests:

• Spirometry measures how well the lungs exhale (breathe out).
• Lung volume measures how well the lungs inhale (breathe in).
• Tests of the diffusion capacity for carbon monoxide (DLCO) show how efficiently the lungs can transfer a gas into the bloodstream.

Why are these tests done?

Pulmonary function tests help:

• diagnose diseases of the lung such as asthma, chronic bronchitis, and emphysema
• determine the cause of shortness of breath
• measure the effects of exposure to chemicals, coal dust, and other toxins on your lung function
• measure the effectiveness of medicines and other treatments.

PFTs can help detect lung disease at an early stage, before you have symptoms.

How do I prepare for these tests?

Eat a light meal and do not smoke for 4 to 6 hours before your test. If you have asthma, ask your healthcare provider if you need to stop using asthma medicine before the test.

How is the test done?

• Spirometry. You breathe into a mouthpiece that is connected to an instrument called a spirometer. The spirometer measures the volume of air that you can force out of your lungs in 1 second after having inhaled as much air as you can. You will be asked to hold the tube of the spirometer in your mouth, breathe in as much air as possible, and then blow out as hard as you can into the spirometer for 1 second. The amount of air you can force out is called your forced expiratory volume, or FEV1.
• Lung volume. For this test, you breathe nitrogen or helium gas through a tube for a certain amount of time. Then the concentration of the gas in a chamber attached to the tube is measured.
• Diffusion capacity. You breathe carbon monoxide for a very short time (often 1 breath). The concentration of carbon monoxide in the air you exhale is then measured. The difference in the amounts of carbon monoxide inhaled and exhaled shows how well gas can travel from your lungs into the blood.

PFTs are not painful, and you will have time to rest between the different breathing tests. The tests may be repeated 2 or more times.

How will I get the test results?

Ask your healthcare provider when and how you will get the results of your tests.

What do the test results mean?

If you do not have lung disease, you will be able to blow out 80% or more of the air in your lungs in 1 second. If you have lung disease, it may take you longer to get all of the air out of your lungs. For example, you may be able to blow out only 20% to 40% of the air in your lungs in 1 second.

PFTs may show what type of lung disease you have.

• If you have obstructive lung disease (emphysema, chronic bronchitis, or asthma), the amount of air you can exhale is decreased, but usually lung volume is normal.
• If you have restrictive lung disease (such as asbestosis, pulmonary fibrosis, or sarcoidosis), the lung volume is decreased while the ability to exhale is normal.

A low diffusion capacity may be a sign of emphysema. It may also be caused by a restrictive lung disease that thickens the lung membrane.

What if my test results are not normal?

Test results are only one part of a larger picture that takes into account your medical history and current health. Sometimes a test needs to be repeated to check the first result. Talk to your healthcare provider about your results and ask questions.

If your test results are not normal, ask your healthcare provider:

• if you need additional tests
• what you can do to work toward normal values
• when you need to be tested again.

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