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
• Reduced lung volume (capacity)

There are no significant risks.

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

Diffusing capacity; DLCO test

Bhakta NR, Kaminsky DA. Pulmonary function testing: Physiologic and technical principles. In: Broaddus VC, Ernst JD, King TE, et al, eds. Murray and Nadel's Textbook of Respiratory Medicine. 7th ed. Philadelphia, PA: Elsevier; 2022:chap 31.

Scanlon PD, Niven AS. Respiratory testing and function. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 73.

Updated by: Denis Hadjiliadis, MD, MHS, Paul F. Harron Jr. Professor of Medicine, Pulmonary, Allergy, and Critical Care, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.