(Braunwald, 15/e, pp 1450-1451.) Carbon monoxide (CO) diffusing capacity provides an estimate of the rate at which oxygen moves by diffusion from alveolar gas to combine with hemoglobin in the red blood cells. It is interpreted as an index of the surface area engaged in alveolar-capillary diffusion. Measurement of diffusing capacity of the lung is done by having the person inspire a low concentration of carbon monoxide. The rate of uptake of the gas by the blood is calculated from the difference between the inspired and expired concentrations. The test can be performed during a single 10-second breath holding or during 1 minute of steady-state breathing. The diffusing capacity is defined as the amount of carbon monoxide transferred per minute per millimeter of mercury of driving pressure and correlates with oxygen transpo from the alveolus into the capillaries. Primary parenchymal disorders, anemia, and removal of lung tissue decrease the diffusing capacity. Conversely, polycythemia, congestive hea failure, and intrapulmonary hemorrhage tend to increase the value for diffusing capacity. In this patient, the possibility of Goodpasture syndrome would be considered.