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 inhaled oxygen 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?
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.
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.