First you should realize that the diffusion (DLCO) test is the most difficult test we do because of the many variables that can modify it. For example, smoking beforehand reduces DLCO, as does anemia; increased cardiac output, such as when someone is excited or really working to do a good test, can increase it. There are many technical variables in measuring it, such as how deep a breath you take in during the test, how long you hold your breath, etc. Many labs don’t reliably control for these factors.
There are two ways to correct the DLCO. One is to correct it for hemoglobin if available. This makes sense as the carbon monoxide is taken up by hemoglobin, and there’s a linear relationship with hemoglobin. But the scatter around this relationship is significant, so it’s an inexact correction.
The other way to correct the DLCO is to consider the lung volume measure at the same time. The volume or area available for diffusion is directly related to the amount that can diffuse across the membrane into the blood. Thus low inspired volume can reduce the diffusion value, yet not reflect the diffusion capacity. But we really lack the data to know how accurate this method is. Diffusion measurements tend to show much more natural variability than our other tests such as spirometry or lung volumes.
My policy is to consider or interpret the absolute, uncorrected diffusion value in light of the clinical situation. If the hemoglobin is low, then the actual diffusion number may be higher. In this case, a value corrected for hemoglobin may be the most useful. I don’t correct for volume, as it’s too inexact.
A decreased diffusion suggests abnormal parenchymal transfer of gases, seen in idiopathic pulmonary fibrosis, some patients with connective tissue diseases, hypersensitivity pneumonitis, and so on. It’s presumably secondary to increased membrane thickness, but we know there are other factors involved. In obstructive lung disease, if associated with increased lung volumes, total lung capacity and residual volume, this suggests emphysema with a decreased diffusion secondary to a loss of lung area for diffusion due to alveoli destruction. Diffusion can be down secondary to air trapping in other lung diseases with severe airflow obstruction because of a lack of gas movement into these areas of lung, reducing the area available.