Lung diffusion testing measures how well the lungs exchange gases. This is an important part of lung testing, because the major function of the lungs is to allow oxygen to "diffuse" or pass into the blood from the lungs, and to allow carbon dioxide to "diffuse" from the blood into the lungs.

Diffusing capacity; DLCO test

You breathe in (inhale) air containing a very small amount of a tracer gas, such as carbon monoxide. You hold your breath for 10 seconds, then rapidly blow it out (exhale). The exhaled gas is tested to determine how much of the tracer gas was absorbed during the breath.

• Do not eat a heavy meal before the test.
• Do not smoke for at least 4 - 6 hours before the test.
• If you use a bronchodilator or inhaler medications, ask your health care provider whether or not you can use them before the test.

The mouthpiece fits tightly around your mouth. Clips are put on the nose.

The test is used to diagnose certain lung diseases, and in persons with established lung disease. Repeatedly measuring the diffusing capacity can help determine whether the disease is improving or getting worse.

Normal test results depend on a person's:

• Age
• Gender
• Height
• Hemoglobin (the protein in red blood cells that carries oxygen) level

Abnormal results mean that gases do not move normally across the lung tissues into the blood vessels of the lung. This may be due to lung diseases such as:

• Emphysema
• Interstitial fibrosis
• Pulmonary hypertension
• Sarcoidosis

There are no significant risks.

Other pulmonary function tests may be done together with this test.

Reynolds HY. Respiratory structure and function: mechanisms and testing. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 85.