 Diffusion capacity of the lung for carbon monoxide DLCO is the clinically available method of assessing gas exchange for practitioners. It is frequently used as a tool for serial monitoring of diffuse parenchymal lung disease. American Thoracic Society has endorsed its use in the assessment of patients with idiopathic interstitial pneumoniasAmerican Thoracic SocietyEuropean Respiratory Society International Multidisciplinary Consensus Classification of the Idiopathic Interstitial Pneumonias . This Joint Statement of the American Thoracic Society ATS, and the European Respiratory Society ERS was adopted by the ATS Board of Directors, June 2001 and by The ERS Executive Committee, June 2001 277-304. 