Diffusing Capacity of the Lungs

alveoli surrounded by capillaries
What is diffusing capacity of the lungs?. istockphoto.com

Diffusing capacity is a measure of how well oxygen and carbon dioxide are transferred (diffused) between the lungs and the blood.

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.

What Does a Low Diffusing Capacity Mean?

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

  • Diffusing capacity may be low if a 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 be low 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.

How is Diffusing Capacity Tested?

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 a 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, doctors may then determine how much carbon dioxide 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 which may result in a 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. You can learn more about obstructive and restrictive lung diseases here.  

Conditions not related to directly to lung function can also result in 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 resulting in decreased DLCO include:

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
  • Asthma
  • Lung cancer
  • Lung surgery

Other conditions which decrease the surface area of the alveoli-capillary membrane

  • Pulmonary embolism
  • Pulmonary hemorrhage
  • 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.

Reasons to Perform Lung Diffusion Testing

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

  • Diagnostic - Doctors 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 doctors determine (along with other factors) how well someone will tolerate surgery.


National Institute of Health. Medline Plus. Lung Diffusion Testing. Updated 11/19/15. https://medlineplus.gov/ency/article/003854.htm

McCormack, M. Diffusing capacity for carbon monoxide. UpToDate. Updated 02/10/16. http://www.uptodate.com/contents/diffusing-capacity-for-carbon-monoxide

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