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Lung Diffusion Tests


Updated July 29, 2008

Definition: Lung diffusion tests help your health care provider determine how well oxygen passes from the alveoli, or air sacs, of your lungs to your bloodstream. The tests measure the "diffusing capacity of the lungs for carbon monoxide" or DLCO.

How is the Test Performed?

The test is performed by an experienced technician. You will inhale a small amount of carbon monoxide gas, hold your breath for 10 seconds, and then exhale as fast as possible. The exhaled gas is then analyzed to determine how much carbon monoxide was absorbed by your body during the breath.

How to Prepare for the Test

Preparation for lung diffusion testing is simple:

  • Do not eat a heavy meal prior to the test
  • Refrain from smoking at least 4 to 6 hours before the test
  • If you use bronchodilators or other inhalers, be sure to check with your health care provider if you should use them prior to the test.

What Happens During the Test?

The test is performed while you are sitting down. A mouthpiece will be fitted tightly around your mouth. Clips will be placed on your nose to pinch it closed so you cannot breath through it.

Why Do I Need the Test?

Health care providers use the test to diagnose the presence of lung disease and to determine the extent of the damage, should lung disease exist.

Normal vs Abnormal Results

Normal results are set to a standard, based on a person's age, sex and height.

Abnormal results mean that gases are not moving the way they should be across your lung tissue to your blood, which may indicate the presence of lung disease. Talk to your doctor for more information.

Risks Involved With the Test

There are no significant risks involved with lung diffusion tests.


In order for your health care provider to make an accurate diagnosis, other pulmonary function tests may also be ordered in adjunct with lung diffusion tests.

Also Known As: DLCO; Diffusing capacity
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