Clinical interpretation of DLCO and KCO: From rationale to clinical and research applications

Abstract

Single-breath methods for measuring the diffusing capacity of the lung for carbon monoxide (DL_CO), Krogh's constant for CO (K_CO), and alveolar volume (V_A) play clinically vital roles in assessing lung diffusion. While the methodology is valid for the kinetics of normal lungs, appropriate interpretations are necessary for lung diseases involving emphysema and/or ventilation heterogeneity. Severe airflow limitations and ventilation heterogeneities lead to the underestimation of lung volume, calculated as V_A, relative to the total lung capacity assessed using the helium closed-circuit method. Notably, the relative increase in K_CO (DL_CO/V_A) compared to DL_CO—resulting from increased blood flow per alveolar–capillary unit in small lungs with fibrosis—is a distinct feature of interstitial lung disease. Therefore, the combined assessment of DL_CO and K_CO may help elucidate the pathophysiology of emphysema and/or pulmonary fibrosis. This review aims to explain Krogh's equation, the difference between DL_CO and D’L_CO, the kinetics, and the clinical application of DL_CO (or D’L_CO) and K_CO. Pulmonary function varies among ethnicities and races; thus, reference equations derived while considering anthropological traits are necessary. Additionally, the link between physiological theory, radiological findings, and the clinical relevance of DL_CO and K_CO is discussed, mostly based on Japanese studies. In this review, DL_CO obtained from the single-breath method is referred to as ”D’L_CO”; however, for convenience, it is described as “DL_CO,” with the term “D’L_CO” used only where necessary.