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.

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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