Gas Diffusion Study


What is a gas diffusion study?

A gas diffusion study is one of a number of pulmonary function tests that help determine how well the lungs are functioning. The word “pulmonary” means having to do with the lungs. “Diffusion” in this case refers to how well the alveoli, or air sacs in the lungs, are delivering oxygen to and removing carbon dioxide from the blood in the capillaries (tiny blood vessels) that surround them.

Other names for this particular test are lung diffusion testing or diffusing capacity of the lung for carbon monoxide (DLCO).

When is a gas diffusion study needed?

There are a number of reasons to conduct a gas diffusion study. It can be used to:

  • Find the reason for a patient’s breathing problems, such as shortness of breath, or other lung-related symptoms such as chest tightness or chest pain; bouts of coughing or wheezing, and an inability to walk or run at the same pace as others in the same age group.
  • See if there is any current lung damage.
  • Screen a person at risk for lung disease. Risk factors include the presence of asthma or allergies; heart disease; smoking; exposure to tuberculosis, and long-term exposure to asbestos, fumes or dusty conditions.
  • Determine a person’s risk level before undergoing surgery.
  • Assess one’s health before beginning a strenuous exercise program.
  • Measure the effect of an already known disease on lung function.
  • Provide information on how well a current treatment may be working.
  • Help track whether an existing condition is getting better or worse.

Test Details

How is a gas diffusion study done?

A gas diffusion study is usually conducted in a special lung function laboratory or clinic. Before coming in for the test, the patient will be given some directions:

  • If any inhaler medicines or bronchodilators are being used to treat breathing difficulties, they should be discontinued for a time before the test is given.
  • Do not eat a heavy meal before the test.
  • Do not smoke 4 to 6 hours before the test.

The procedure for taking the test is as follows:

  • The patient will be asked to sit or stand next to a measuring device called a spirometer. This small machine measures the volume of air entering and leaving the lungs, and is used in several types of lung function tests.
  • A clip is placed on the patient’s nose and a mouthpiece is fitted around the mouth so that it is possible to breathe only through a tube connected to the mouthpiece. It is important to keep a tight seal with the mouth when performing this test.
  • The patient breathes in a gas mixture.
  • The breath is held for 10 seconds then blown out into the spirometer tube.
  • The machine measures how much of the tracer gas there is in the air mixture being exhaled, indicating how much of the gas the lungs were able to absorb.

Either before or after the gas diffusion test, it is possible that another type of pulmonary function test (PFT) may be performed if recommended by the patient’s doctor. There is no reason why more than one PFT cannot be conducted in the same session.

Other types of PFTs include spirometry and lung volume measurements (by dilution or body plethysmography).

What are the risks of having a gas diffusion study?

Taking part in a gas diffusion study rarely causes any problems. In some people, a feeling of lightheadedness might occur from holding the breath, but this feeling should be only temporary. There is no harm from inhaling the gas mixture.

Results and Follow-Up

What determines the results of a gas diffusion study?

The diffusion rate is determined by a number of factors, including:

  • The amount of surface area over which diffusion can occur
  • The blood volume in the capillaries
  • The concentration of hemoglobin (the red part of blood cells whose job is to transport oxygen) present in the blood
  • The thickness of the membrane between the alveoli and the capillaries
  • The presence of excess fluid in the alveoli

When a lung disorder affects any of the above, a low rate of diffusion can occur and the person is left with less than maximum oxygen uptake.

What could abnormal results of a gas diffusion study indicate?

Obstructive lung disease

  • Cystic fibrosis (overproduction of mucus, leading to chronic infections in the lungs)
  • Emphysema (an abnormal increase in the size of the air spaces in the lungs, leading to difficult breathing and infection)

Damage to lung tissue

  • Drug reactions
  • Asbestosis or other lung disease caused by repeated exposure to airborne fibers
  • Inflamed alveoli due to allergic reactions
  • Sarcoidosis (clumps of abnormal tissue)

Systemic diseases that affect the lungs

Heart conditions

  • Pulmonary thromboembolism (blockage of a blood vessel in the lung)
  • Heart attack
  • Mitral stenosis (narrowing of the mitral valve of the heart)
  • High blood pressure
  • Pulmonary edema (fluid in the lungs )


Sometimes a gas diffusion study will show a diffusion rate that is higher than normal. In such cases, the following conditions could play a part:

  • Diseases associated with increased blood flow in the lungs
  • Polycythemia (abnormally high number of red blood cells)
  • Exercise
  • Bleeding in the lungs

Last reviewed by a Cleveland Clinic medical professional on 09/27/2018.


  • Enright MP. Office-based DLCO tests help pulmonologists to make important clinical decisions. Respir Investig. 2016;54(5):305-11.
  • National Heart, Lung, and Blood Institute. Pulmonary Function Tests. ( Accessed 9/28/2018.
  • Miaskiewicz, Joseph J.. "Pulmonary Function Testing." Principles and Practice of Hospital Medicine, 2e McKean SC, Ross JJ, Dressler DD, Scheurer DB. McKean S.C., Ross J.J., Dressler D.D., Scheurer D.B. Eds. Sylvia C. McKean, et al. New York, NY: McGraw-Hill.

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