Incorporating Lung Diffusing Capacity for Carbon Monoxide in Clinical Decision Making in Chest Medicine J. Alberto Neder, MD, PhD, FRCPC, FERS a, *, Danilo C. Berton, MD, PhD b , Paulo T. Muller, MD, PhD c , Denis E. O’Donnell, MD, FRCPI, FRCPC, FERS d INTRODUCTION The transfer of oxygen (O 2 ) from atmospheric air to pulmonary capillaries, and carbon dioxide (CO 2 ) in the opposite direction, is the key task of the lungs in terrestrial animals. Because most respiratory diseases in humans impair the efficiency of the lungs as gas exchangers, there is a strong ratio- nale for pulmonary function tests (PFTs) aimed at exploring the integrity of such a crucial endeavor. 1 Clinically, this is more commonly carried out by tests of lung diffusing capacity (DL) 2 using a highly diffusible gas such as carbon monoxide (CO). 3 More than 100 years ago, 4 Marie Krogh used the Disclosure Statement: No author has any relationship with a commercial company that has a direct financial interest in the subject matter or materials discussed in this article or with a company making a competing product. a Laboratory of Clinical Exercise Physiology, Division of Respirology and Sleep Medicine, Department of Med- icine, Kingston Health Science Center, Queen’s University, Richardson House, 102 Stuart Street, Kingston, Ontario K7L 2V6, Canada; b Division of Respirology, Federal University of Rio Grande do Sul, Porto Alegre, Brazil; c Division of Respirology, Federal University of Mato Grosso do Sul, Campo Grande, Brazil; d Respiratory Investigation Unit, Division of Respirology and Sleep Medicine, Kingston Health Science Center & Queen’s University, Kingston, Ontario, Canada * Corresponding author. E-mail address: alberto.neder@queensu.ca KEYWORDS  Lung function  Lung diffusing capacity  Gas exchange  Dyspnea  Hypoxia KEY POINTS  Measurements of single-breath lung diffusing capacity for carbon monoxide (DLCO) provide an in- tegrated picture of the complex mechanisms involved in the transfer of oxygen from atmospheric air to lung capillaries.  To maximize the clinical information derived from those measurements, DLCO should be analyzed taking into consideration the ratio between total lung capacity and the accessible alveolar volume (TLC/VA ratio), VA, and the diffusion coefficient for carbon monoxide (KCO).  Clinical scenarios in which DLCO is more likely to provide relevant information include dyspnea of unknown origin or out-of-proportion dyspnea, investigation of the mechanisms of dyspnea and ex- ercise intolerance in chronic obstructive pulmonary disease (COPD), differential diagnosis between asthma and COPD, investigation of the causes underlying a restrictive ventilatory defect, manage- ment of patients with pulmonary vascular disease and interstitial lung disease, and preoperative assessment. Clin Chest Med 40 (2019) 285–305 https://doi.org/10.1016/j.ccm.2019.02.005 0272-5231/19/Ó 2019 Elsevier Inc. All rights reserved. chestmed.theclinics.com