Lung diffusion testing

You breathe in (inhale) air containing a very small amount of carbon monoxide and a tracer gas, such as methane or helium. 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.

Before taking this test:

• Do not eat a heavy meal before the test.
• Do not smoke for at least 4 to 6 hours before the test.
• If you use a bronchodilator or other inhaled medicines, 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 your nose.

The test is used to diagnose certain lung diseases, and to monitor the status of people 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
• Sex
• 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:

• COPD
• Interstitial fibrosis
• Pulmonary embolism
• Pulmonary hypertension
• Sarcoidosis
• Bleeding in the lung
• Asthma

There are no significant risks.

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

Gold WM, Koth LL. Pulmonary function testing. In: Broaddus VC, Mason RJ, Ernst JD, et al, eds. Murray and Nadel's Textbook of Respiratory Medicine. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 25.

Scanlon PD. Respiratory function: mechanisms and testing. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 85.