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 (DLCO) 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 DLCO, and sometimes, the DLCO corrected by alveolar volume (VA) at the measurement. However, DLCO is the product of two independent components in reality-the transfer coefficient for carbon monoxide (KCO) and VA. Conventionally, KCO has often been expressed as DLCO/VA. In this report, we will demonstrate the change in relationship of DLCO with KCO 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 "KCO" is preferable to DLCO/VA to avoid misleading.