The Utility of DLCO

DLCO ("Diffusing capacity of the Lungs for CO"), also known as "transfer factor of the lungs for CO", is a key metric in the evaluation of pulmonary diseases, and is often involved in the work-up of respiratory symptoms like dyspnea. Part of spirometry testing, its key purpose is to evaluate the function of gas exchange across the alveolar-capillary membrane, using CO as the proxy molecule standing in for other respiratory gases like oxygen. CO is a gas that diffuses across the alveolar-capillary membrane and binds with high affinity to hemoglobin. This high affinity makes CO diffusion more independent of cardiac output and metabolic rate than oxygen. 

One useful application of DLCO is its use in differentiating the types of restrictive and obstructive lung diseases. Since DLCO reflects the properties of the alveolar-capillary membrane, a decrease in the rate of CO diffusion would indicate a change in the gas-exchanging portions of the lung parenchyma. A normal DLCO would suggest extrapulmonary causes of disease. Thus, the DLCO distinguishes between COPD (where the surface area of alveoli is reduced) and asthma (transient constriction of conducting airways) in patients with obstructive patterns on spirometry, while it distinguishes between interstitial disease (thickened alveolar-capillary membranes) and thoracic wall etiologies (extrapulmonary causes) in patients with restrictive patterns.