Diffusing Capacity

The diffusing capacity (DL_CO) is the part of a comprehensive series of tests (the pulmonary function tests) that is done to determine the overall ability of the lung to transport gas into and out of the blood. DL_CO is reduced in certain diseases of the lung and heart. This test has been standardized according to a position paper by a task force of the European Respiratory and American Thoracic Societies.

In respiratory physiology, the diffusing capacity has a long history of great utility, but the words themselves are now misleading because they are archaic: neither is diffusion measured nor is the value obtained from this test a capacity nor even a capacitance, but in fact a conductance. While the term diffusing capacity is retained in the United States for reasons of historical continuity, terminology using transfer factor is now preferred in Europe and elsewhere. Nevertheless, there are 7 - 8 times more citations for the original terminology in PubMed, so any change in usage will be slow to happen.

Worse still, the term diffusing capacity is positively misleading, since gas transport is not diffusion limited in all but the most extreme cases, such as for oxygen uptake at very low ambient oxygen or at very high pulmonary blood flow. Critics of the term "diffusion capacity" argue that it may be misleading for other reasons as well, and point out two problems with the term. The first is that the test measures not just diffusion across the alveolar-capillary membrane, but also takes into account factors affecting the chemical combination of a given gas with hemoglobin. The second criticism is that the test is typically measured under submaximal conditions and doesn't truly reflect a functional capacity. For these reasons the term "transfer factor" has been proposed to better reflect the physiological process being measured.

Finally, the diffusing capacity does not directly measure the primary cause of hypoxemia, or low blood oxygen, namely mismatch of ventilation to perfusion:

• Not all pulmonary arterial blood goes to areas of the lung where gas exchange can occur (the anatomic or physiologic shunts), and this poorly oxygenated blood rejoins the well oxygenated blood from healthy lung in the pulmonary vein. Together, the mixture has less oxygen than that blood from the healthy lung alone, and so is hypoxemic.
• Similarly, not all inspired air goes to areas of the lung where gas exchange can occur (the anatomic and the physiological dead spaces), and so is wasted.