What Is Lung Diffusion Testing?

If your doctor needs to figure out why you're having trouble breathing, or to check for other lung problems, they may suggest lung diffusion testing. It measures how well your lungs are working.

Lung diffusion is your ability to pass oxygen into the blood from the air sacs of the lungs, and pass carbon dioxide (CO2) back into the lungs from the blood.

Lung diffusion testing measures how well your lungs do this back-and-forth exchange. You may hear your doctor call this test by other names, such as:

  • Gas diffusion study
  • Diffuse capacity test
  • Diffusing capacity of the lung for carbon monoxide (DLCO)

What to Expect Before the Test

Your doctor may ask that in the hours before the test, you don't:

What to Expect During the Test

You typically go to a lung clinic for lung diffusion testing. A nurse will attach a mouthpiece that seals tightly around your mouth.

They also will put clips on your nose to stop any air from going in or out. The mouthpiece connects through a tube to a machine called a spirometer that measures the amount of air you breathe in and out.

You'll breathe in a special gas mixture and hold it for 10 seconds before you blow it out into the spirometer. The mixture you breathe in has a small amount of carbon monoxide along with gas "tracers" like methane or helium, which a machine measures after you breathe out to see how much your lungs absorbed.

The amount that remains tells your doctor important information about how well your lungs put oxygen into your blood and take out CO2.

Why You Might Need the Test

There are lots of reasons you might get lung diffusion testing. Your doctor might need to:

  • Look for signs of suspected lung damage
  • Help diagnose the cause of breathing problems
  • Track the progress of a current illness
  • Test how well treatment is working
  • Check your lung health before surgery
  • Screen you if you're at risk for lung disease because of smoking, heart problems, etc.


What Your Results Mean

Your doctor will consider different results "normal" depending on your:

  • Sex
  • Height
  • Age
  • Level of hemoglobin (protein in red blood cells)
  • Other health problems you may have

An "abnormal" result means gases don't move back and forth through your lung tissue as easily as they should. This may be a sign of lung problems like:

Your doctor will talk to you about your results, what they could mean, and what you should consider doing next.

WebMD Medical Reference Reviewed by Arefa Cassoobhoy, MD, MPH on July 28, 2020



CDC: "Spirometry Reference Value Calculator."

Cleveland Clinic: "Gas Diffusion Study."

Medscape: "How is a diffusing capacity of lung for carbon monoxide (DLCO) test performed?"

UCLA Health: "Lung diffusion testing."

UpToDate: "Multidimensional assessment of COPD," "Chronic obstructive pulmonary disease: Definition, clinical manifestations, diagnosis, and staging."

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