In which condition would you expect to see decreased diffusing capacity?

Decreased diffusing capacity is commonly associated with interstitial lung disease due to the involvement of the lung's interstitium, which can lead to thickening of the alveolar-capillary membrane. This thickening can impair the diffusion of gases, decreasing the lungs' ability to transfer oxygen into the bloodstream effectively. In interstitial lung disease, various pathological processes such as inflammation, fibrosis, or edema contribute to this reduction in diffusivity.

In contrast, conditions like asthma or COPD might impact airflow and cause dynamic changes in lung volumes, but they do not typically lead to a significant reduction in diffusing capacity unless there is an advanced form of the disease where lung parenchyma is also involved in a more significant manner. Active respiratory infections may affect ventilation and might cause transient changes in gas exchange, but they do not generally create a long-term decrease in diffusing capacity like interstitial lung disease does.