Diffusing Capacity of the Lungs Test (DLCO)

Diffusing capacity is a measure of how well oxygen and carbon dioxide are transferred (diffused) between the lungs and the blood, and can be a useful test in the diagnosis and to monitor treatment of lung diseases. Diffusing capacity can also be important prior to lung surgery as a predictor of how well the surgery will be tolerated. Diffusing capacity may be reduced in a few ways, and healthcare providers usually use the measure along with other pulmonary function tests to diagnose and determine the severity of either restrictive or obstructive lung diseases.

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Reasons to Perform Lung Diffusion Testing

There are 3 primary reasons why your healthcare provider may order lung diffusion testing. These include:

  • Diagnostic: Healthcare providers may use DLCO to diagnose medical conditions such as emphysema
  • Treatment Monitoring: Diffusing capacity may be monitored to determine whether a condition has worsened, or if it has improved with treatment.
  • Pre-Surgical: With lung cancer, diffusing capacity is an important test for people who are considering lung cancer surgery because it can help healthcare providers determine (along with other factors) how well someone will tolerate surgery.

Meaning of a Low Diffusing Capacity

Oxygen and carbon dioxide both need to pass through a thin layer in the lungs called the alveolar-capillary membrane. This is the layer between the small air sacs in the lung (the alveoli) and the smallest blood vessels that travel through the lungs (capillaries).

How well oxygen that is inhaled can pass (diffuse) from the alveoli into the blood, and how well carbon dioxide can pass from the blood capillaries into the alveoli and be exhaled, depends on how thick this membrane is, and how much surface area is available for the transfer to take place.

There are two separate mechanisms by which diffusing capacity may be reduced.

  • Diffusing capacity may be low if lung disease is present that causes the membrane to be thicker, for example, in diseases such as pulmonary fibrosis and sarcoidosis.
  • Diffusing capacity may also below if there is less surface area available for the transfer of oxygen and carbon dioxide, for example, with emphysema or if a lung or part of a lung is removed for lung cancer.

Testing Diffusing Capacity

Testing for diffusing capacity is often done along with other pulmonary function tests. In this test, a mask is placed over your face. During the test, you will take in a deep breath of gas, hold your breath, and then the air that you exhale will be measured.

The gas you breathe in will contain carbon monoxide as well as a tracer gas such as helium. Note, that these are inhaled in small amounts and this is not a dangerous test. When the exhaled gas is exhaled, healthcare providers may then determine how much carbon monoxide and helium diffused across the alveoli into the capillaries, by determining the difference between that which is inhaled and that which is exhaled.

This test is often referred to as DLCO — which stands for diffusion across the lungs of carbon monoxide.

Causes of Low Diffusing Capacity

There are several conditions that may result in low diffusing capacity. Restrictive lung diseases such as pulmonary fibrosis most often decrease diffusing capacity (DLCO) because of scarring and thickening of the area between the alveoli and capillaries.

 In contrast, obstructive lung diseases such as emphysema may decrease DLCO by reducing the surface area through which gas can be exchanged.

Conditions not related directly to lung function can also result in a decreased surface area available between the alveoli and capillaries. For example, a blood clot in an artery in the lungs (pulmonary embolism) may result in carbon monoxide brought into the alveoli being unable to be transferred to the capillaries which the artery supplies. 

Diseases Associated With a Low Diffusing Capacity

Understanding a low diffusing capacity requires looking at the differences between obstructive and restrictive lung diseases and how these affect lung function.

Restrictive Lung Diseases Causing Thickening of the Alveolar-Capillary Membrane

  • Pulmonary fibrosis
  • Sarcoidosis

Obstructive Lung Diseases and Diseases Causing Less Surface Area in the Lungs

  • Emphysema
  • Lung cancer
  • Lung surgery

Other Conditions Which Decrease the Surface Area of the Alveoli-Capillary Membrane

  • Pulmonary embolism
  • Primary pulmonary hypertension

Causes of High Diffusing Capacity

Rarely, DLCO may instead be high. This may occur with asthma, polycythemia vera (a disease with an elevated hemoglobin level), and congenital diseases that cause blood to be shunted from the left side of the heart to the right side of the heart. With these conditions, however, there are often other signs, symptoms, and testing abnormalities that lead to the diagnosis.

A Word From Verywell

Diffusing capacity is but one test that is used to evaluate lung diseases. Though the test (as well as others) can be confusing, learning about the meaning behind the test can help you better understand your disease.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. National Institute of Health Medline Plus. Lung diffusion testing.

  2. Cleveland Clinic. Gas diffusion study: Results and follow-up.

  3. Cleveland Clinic. Gas diffusion study: Test details.

Additional Reading

By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."