Both DLCO and KCO are vital indices in assessment of diffusing capacity for carbon monoxide by the single-breath method

The measurement of diffusing capacity of the lung for carbon monoxide (DL_CO) is an invaluable pulmonary function test for assessing gas exchange in the lungs. Typically, the test is conducted via the single-breath method, wherein after maximum inhalation of carbon monoxide and helium simultaneously, the breath is held for 10 s. The test provides the single value of DL_CO, and sometimes, the DL_CO corrected by alveolar volume (V_A) at the measurement. However, DL_CO is the product of two independent components in reality-the transfer coefficient for carbon monoxide (K_CO) and V_A. Conventionally, K_CO has often been expressed as DL_CO/V_A. In this report, we will demonstrate the change in relationship of DL_CO with K_CO uniquely and characteristically due to emphysema, fibrosis, and their combined disease. Therefore, the assessment of both indices is highly desirable in the precise interpretation of the test. The term "K_CO" is preferable to DL_CO/V_A to avoid misleading.