A test of the diffusing capacity of the lungs for carbon monoxide (DLCO) is one of the most clinically valuable tests of lung function. DLCO measures the ability of the lungs to transfer gas from inhaled air to the red blood cells in pulmonary capillaries. The test is rarely available in the outpatient office setting because the instrument costs more than $20,000 and requires much more skill and experience for its use and maintenance than does a spirometer. (See "Overview of pulmonary function testing in adults" and "Office spirometry".)
However, the DLCO test is convenient and easy for the patient to perform. The ten seconds of breathholding required for the DLCO maneuver is easier for most patients to perform than is the forced exhalation required for spirometry. Completion of two or three DLCO maneuvers also usually takes less time than pre- and post-bronchodilator spirometric tests.
Standards for DLCO instruments, performance of the test, and calculation of the results were initially published by the American Thoracic Society in 1987, and updated in 2005 [1,2]. The indications for DLCO measurement and the interpretation of the results will be discussed here.
The DLCO is designed to reflect properties of the alveolar-capillary membrane, specifically the ease with which oxygen moves from inhaled air to the red blood cells in the pulmonary capillaries. The uptake of most soluble gases (such as nitrous oxide or acetylene) is limited by (and varies with) pulmonary blood flow. In contrast, the strong affinity of hemoglobin for carbon monoxide (CO), combined with the enormous capacity of the red cell mass to absorb CO, make the uptake of CO less dependent on cardiac output. Thus, diseases in which the uptake of oxygen is reduced cause parallel decreases in the uptake of CO, as measured by the DLCO.
Older textbooks suggest that thickening of the alveolar-capillary membrane (in interstitial lung disease) and loss of alveolar membrane surface area (in emphysema) are the primary causes of a low DLCO. However, subsequent experimental data suggest these and most other diseases that influence the DLCO do so by reducing the volume of red blood cells in the pulmonary capillaries. The total volume of blood in the lungs in healthy adults at rest is less than 150 mL; the volume of blood in the pulmonary capillaries and the DLCO are increased in the following circumstances: