A measurement of carbon monoxide (CO) transfer from inspired gas to pulmonary capillary blood. During the test, the subject inspires a gas containing CO and one or more tracer gases to allow determination of the gas exchanging capability of the lungs. The trait is EFO_0009369 (diffusing capacity of the lung for carbon monoxide). Also known as: carbon monoxide diffusing capacity, pulmonary diffusing capacity, transfer factor of the lung for carbon monoxide.