Lung Diffusion Testing (DLCO)

A lung diffusion test (DLCO) is a type of pulmonary function test that tells your provider how well your lungs are working. It uses a gas containing carbon monoxide to measure how much oxygen moves from your lungs to your blood when you inhale. Providers use a DLCO test to help diagnose lung conditions and measure how well treatments are working.


What is lung diffusion testing (DLCO)?

Lung diffusion testing determines how well oxygen moves from your lungs to your blood. It’s a type of pulmonary function test. It helps your healthcare provider understand how well your lungs are working.

During the test, you breathe in a gas that contains a small amount of carbon monoxide. The test measures how much carbon monoxide passes from your lungs to your blood, or the “diffusing capacity for carbon monoxide” (DLCO for short).

What is diffusion?

When you breathe air in, the oxygen travels through your airways into small air sacs (alveoli). From there, your blood picks up the oxygen as it travels through nearby blood vessels (capillaries). Your blood delivers the oxygen to your tissues to use for energy. A number of factors determine how well oxygen transfers from your lungs to your blood (diffusion), including:

  • The amount of surface area in your alveoli.
  • The amount of blood in your capillaries.
  • The concentration of hemoglobin (a protein that carries oxygen) in your blood.
  • The thickness of the membrane between your alveoli and capillaries.
  • Excess fluid in your alveoli.

What is lung diffusion testing (DLCO) used for?

Lung diffusion testing can be used to:

  • Understand what’s causing certain symptoms such as shortness of breath, coughing or wheezing.
  • Assess lung damage.
  • Screen for certain lung diseases.
  • Evaluate how well your lungs are working before surgery or beginning a strenuous exercise program.
  • Understand how an underlying disease is affecting your lung function.
  • Get information on how well a treatment is working.
  • Help track whether an existing condition is getting better or worse.


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Test Details

How does a DLCO test work?

A DLCO test uses a special mixture of gas to measure how well oxygen and carbon dioxide are moving between your lungs and your blood. During the test, you breathe in a gas that’s a mixture of nitrogen and oxygen (which naturally make up most of the air you breathe), plus:

  • A small amount of carbon monoxide (not enough to hurt you), which should move easily from your lungs to your blood.
  • A small amount of a tracer gas, like methane. A tracer gas isn’t absorbed into your blood and helps measure the amount of carbon monoxide absorbed.

When you breathe in, your blood absorbs some or all of the carbon monoxide from the gas in your lungs. When you breathe out into the machine, it measures how much carbon monoxide your blood absorbed based on how much is left in the gas you breathed out. This measurement can give your provider clues as to how well your lungs are working.

How do I prepare for lung diffusion testing?

Your provider might give you instructions on how to prepare for a DLCO test. They could include:

  • Don’t smoke four to six hours before the test.
  • Don’t use supplemental oxygen 10 minutes before the test (only if you’re able to safely).
  • Don’t exercise right before the test.

What should I expect during a lung diffusion (DLCO) test?

During a DLCO test, a technician will put a clip over your nose. They’ll have you:

  1. Put your mouth over a mouthpiece attached to a machine. The machine will deliver the gas mix. It’ll also measure and record your results throughout the test.
  2. Take a few normal, steady breaths.
  3. Inhale deeply and exhale completely.
  4. Breathe in deeply through your mouth.
  5. Hold your breath for 10 seconds.
  6. Breathe out.

Your technician will wait a few minutes and repeat the test at least one more time.

What are the risks and side effects of a DLCO test?

The risks of a DLCO test are very minimal. Some people might get light-headed during the test. The amount of carbon monoxide is very small and not enough to hurt you.


Results and Follow-Up

What do the results of a DLCO test mean?

The results of a lung diffusion test are given in a percentage of what they expect your DLCO to be (predicted value). Not everyone’s expected DLCO value is the same. It’s based on factors like age and sex. Your results could be:

  • Normal DLCO: Between 75% and 140% of the predicted value.
  • Mildly reduced DLCO: 60% to 75% or the lower limit of normal (LLN) predicted value.
  • Severely reduced DLCO: Less than 40% of the predicted value.

What does low DLCO mean?

Low DLCO means your lungs aren’t getting oxygen from the air to your blood efficiently. This might be because of issues in your heart or your lungs. For example, your heart might not be pumping well enough, or your blood isn’t able to effectively pick up oxygen.


What causes low DLCO?

Many things can cause a low DLCO. Some specific causes include:

What causes high DLCO?

Some conditions cause you to have a higher DLCO than expected. These include:

If the results are abnormal, what are the next steps?

Your provider will use the results of the test along with other lung function test results (like spirometry) to diagnose you. They may need additional tests or imaging to more fully understand what’s causing abnormal lung diffusion.

When should I call my doctor?

Talk to your healthcare provider if you have any questions about the test or the results.

A note from Cleveland Clinic

Lung diffusion testing (DLCO) is a safe, noninvasive way for your healthcare provider to get important information about how well your lungs are working. They can use it to help diagnose conditions, to evaluate how well treatments are working and to clear you for surgery. Don’t hesitate to ask your provider any questions you have about how the test works, why you’re doing it and what the results mean.

Medically Reviewed

Last reviewed on 02/08/2024.

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