DLCO testing evaluates diffusion capacity for which gas?

• A. Oxygen
• B. Carbon monoxide
• C. Nitrogen
• D. Helium

Answer: (B)

DLCO measures how well the lungs transfer gas from the air into the blood. Carbon monoxide is used because it binds to hemoglobin with very high affinity. When CO is inhaled, its uptake by the blood is driven mainly by the alveolar-capillary membrane’s diffusion capacity and the capillary blood volume, since CO rapidly binds Hb and keeps a strong gradient for transfer. This makes the measured uptake a good reflection of diffusion across the alveolar-capillary barrier, largely independent of ventilation. Gases like nitrogen or helium don’t bind to Hb and wouldn’t provide a clean diffusion measurement, and oxygen diffusion is influenced by factors that make interpretation more complex. So the diffusion capacity assessed by DLCO specifically reflects how well carbon monoxide can diffuse across the lung membrane into the blood.