Doctors use lung diffusion tests to measure how well a person’s lungs exchange oxygen and carbon dioxide. The test can detect lung damage and help diagnose lung conditions, including emphysema and pulmonary hypertension.

During the test, a person breathes in a special gas and exhales into a tube. A doctor will analyze samples of exhaled air to see how well the lungs are taking in oxygen and expelling carbon dioxide. Doctors call this process diffusion.

This article provides an overview of the lung diffusion test. It looks at when doctors use it, the procedure, and what the results mean.

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A doctor may use lung diffusion testing to diagnose asthma, interstitial fibrosis, and lung hemorrhage.

The lung diffusion test is one type of pulmonary function test. The test is quick and harmless.

The lungs function by taking in oxygen during inhalation. The body then exchanges the oxygen for carbon dioxide, which is a waste product of respiration and exhales it.

If the lungs become damaged, they may not be able to diffuse gases as efficiently. As a result, a person may have more carbon dioxide in their lungs than usual, and they may be less able to accept oxygen when inhaling.

Lung diffusion tests check for lung damage by measuring how well the lungs exchange gases.

During a lung diffusion test, a person will inhale a small amount of air that contains a small amount of carbon monoxide and a tracer gas — often methane or helium — then exhale into a tube.

The doctor collects and tests the exhaled air to see how much gas a person has breathed out. This shows how well the body has absorbed the tracer gas. This test gives the doctor an idea of how much oxygen a person breathes in and how well the gas exchange system is working.

This test is also known as diffusion lung capacity of carbon monoxide (DLCO) test.

Doctors test a person’s lung diffusion capacity to help diagnose lung-related conditions. If a person already has a lung disease, they may use the test to determine how well current treatments are working.

Doctors may use lung diffusion testing to diagnose or monitor a range of conditions, including:

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A doctor may recommend avoiding particular activities or medication before a lung diffusion test.

Lung diffusion tests are noninvasive and require little preparation. However, always follow the doctor’s instructions about how to prepare for any medical test.

Doctors may recommend:

  • Taking medications as usual throughout the day.
  • Refraining from smoking on the day of the test.
  • Refraining from using some short-acting medications, such as inhalers, before the test. However, this may not always be possible.
  • Avoiding eating or drinking for several hours before the test, though this is not always necessary.

If it is safe, a doctor may recommend that a person stops using supplemental oxygen for at least 10 minutes before the test starts. An oxygen mask can affect the results by increasing oxygen levels and reducing how well carbon monoxide sticks to red blood cells.

It is a good idea for people to ask their doctor whether they should avoid doing particular activities or taking specific medications before a lung test.

During the procedure, the person will breathe into a mask that fits snugly over their mouth. They will have a clip on their nose to ensure that all of the air they inhale and exhale comes from the apparatus.

The procedure involves the following actions:

  • quickly inhaling a specific gas to total lung capacity
  • holding the breath for 10 seconds
  • exhaling gently into a tube

Different clinics or surgeries may perform the lung diffusion test differently. According to the National Heart, Lung, and Blood Institute (NHLBI), a lung diffusion capacity test can involve a person breathing in and out through a tube for several minutes. In this test, the person does not have to breathe intensely.

The gas people that people inhale during the procedure contains a combination of gases that can have the following concentrations:

  • 0.3% carbon monoxide
  • 21% oxygen
  • 0.3% methane or other tracer gas, such as helium
  • nitrogen

The person exhales into a special device that will measure the amounts of exhaled carbon monoxide and tracer gas.

People may need to repeat the test several times for the most accurate results. They will need to wait at least 4 minutes between readings for the lungs to clear out the previous test gases. People with obstructive lung disease may need to wait for 10 minutes.

A healthcare professional will often draw blood at the same appointment to measure the levels of hemoglobin in a person’s blood. A doctor will use these results to help calculate a person’s DLCO results.

Normal ranges can vary based on several factors, including a person’s:

  • age
  • sex
  • height
  • hemoglobin levels

A doctor will take these factors into account and work out a predicted level of diffusion capacity.

The normal range for DLCO is as follows:

  • 80–120% of its predicted value for men
  • 76–120% of its predicted value for women

Higher or lower readings can indicate that the lungs are not exchanging gases efficiently.

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If the test results are abnormal, a doctor may order another type of test, such as spirometry.

If a person has a condition that affects blood flow to the lungs or they have damaged lung tissue, their lungs will not be able to diffuse gas as well. As a result, they may have a lower or higher lung diffusing capacity measurement than expected.

Low levels can indicate:

High levels can indicate:

  • asthma
  • bleeding in the lungs, or pulmonary hemorrhage
  • a high red blood cell count, or polycythemia

A doctor will assess a person’s risk factors and other symptoms to work out the cause of the abnormal results. They will often order other pulmonary function tests to help with the diagnosis, such as spirometry and lung volume tests.

When a person has a lung diffusion test to monitor an existing condition, the doctor will compare their results to previous tests to see if the condition has improved, worsened, or stabilized. Based on the results, a doctor may continue or change current treatments.

Lung diffusion capacity testing is a quick, noninvasive procedure that can help doctors estimate how well a person’s lungs are exchanging gases. They may use the test to diagnose conditions or to assess how well lung-related treatments are working.

A doctor will often recommend lung diffusion capacity testing in addition to other pulmonary function tests to gain an overall clinical picture of how well a person’s lungs are working.