Diffusion capacity of the lungs (DLCO), also known as the transfer factor is used to assess the lungs' ability to transfer gas from inspired air to the blood stream. It is an important test to determine the ability of the respiratory system for gas exchange.  

DLCO acts as a surrogate marker of the extent of lung damage seen in different clinical conditions such as interstitial lung diseases, pulmonary fibrosis and emphysema (COPD). It is also reduced in pulmonary vascular disease (ie, chronic thromboembolic pulmonary hypertension or idiopathic pulmonary arterial hypertension.

DLCO is commonly measured with single-breath technique with the help of a special equipment through which the patient breathes. The test is usually done with small concentrations of carbon monoxide (in place of oxygen) used as a measure of transfer of gases across the alveolo-capillary membrane.

LCO is also used to assess the disease progress and treatment response in addition to other tests such as spirometry, 6-minute walk test and blood gases.