Which factor is primarily concerning to diffusion when using CO in testing?

The primary concern regarding diffusion when using carbon monoxide (CO) in pulmonary function testing is the condition of the alveolocapillary membranes. These membranes play a crucial role in the gas exchange process, as they separate the air in the alveoli from the blood in the capillaries. For effective diffusion of gases, the membranes must be intact, have adequate surface area, and maintain appropriate permeability.

When evaluating diffusion capacity using CO, any pathological conditions affecting the alveolocapillary membranes can significantly impact the results. For instance, conditions like pulmonary fibrosis, emphysema, or pulmonary edema can thicken the membrane or reduce the surface area available for gas exchange, leading to a reduced diffusion capacity. When the integrity and functionality of these membranes are compromised, the ability of CO to diffuse from the alveoli into the blood will be impaired, resulting in lower diffusion capacity measurements.

While factors like airway resistance, airflow rates, and the quality of the diffusion gas mixture may influence testing, they do not have as direct and significant an impact on diffusion capacity for CO as the condition of the alveolocapillary membranes does. Thus, the condition of these membranes is the primary concern in diffusion testing using CO.