Diffusion Capacity evaluates the Gas Exchange (oxygen and carbon dioxide) : the ability of lungs to transfer gas, from the inhaled Atmospheric Air to the Alveoli and the Circulating Blood, across the Alveolar-Capillary Membrane.

It is based on the inhalation of a specific gas mixture that contains a small amount of carbon monoxide as a tracer. Measurement and Monitoring of Diffusing Capacity over time, is crucial in the diagnosis and monitoring of interstitial lung diseases, such as pulmonary fibrosis and sarcoidosis.