Diffusion capacity testing measures gas transfer from the lungs to the blood. What best describes the test?

• A. The difference between the inhaled and exhaled carbon monoxide is used to estimate transfer from lung to blood.
• B. The rate of blood flow through the lungs using X-ray imaging.
• C. The total lung capacity during maximal inspiration.
• D. The electrical activity of respiratory muscles.

Answer: (A)

Diffusion capacity testing assesses how well gases move from the air in the lungs into the blood, focusing on the transfer across the alveolar-capillary membrane. In this test, a small amount of carbon monoxide is inhaled, then after a brief breath-hold the person exhales. By comparing how much CO is taken up (inhaled vs. exhaled), the test estimates the diffusion capacity of the lungs (DLCO). Carbon monoxide is used because it binds to hemoglobin very quickly, and its uptake is limited by diffusion, making the result reflect the alveolar surface area and membrane thickness rather than blood flow.

Other options describe different measurements: imaging the rate of blood flow through the lungs would be perfusion imaging, not diffusion; measuring total lung capacity is about lung volumes; and assessing electrical activity of respiratory muscles relates to neuromuscular function rather than gas transfer.