Which statement about diffusion capacity testing is true?

• A. It uses carbon monoxide to measure diffusion capacity.
• B. It uses computed tomography to assess diffusion capacity.
• C. It measures only lung volumes.
• D. It is a noninvasive test that monitors airway resistance during sleep.

Answer: (A)

Diffusion capacity testing assesses how well gases move from the airspaces into the blood, reflecting the function of the alveolar–capillary membrane. The standard approach uses carbon monoxide because it binds hemoglobin very readily and at very low concentrations, so the amount taken up by the blood mainly reflects the membrane’s ability to diffuse gas rather than changes in blood flow. By measuring how much CO is absorbed during a controlled breath (or under steady breathing), clinicians calculate the diffusion capacity (DLCO), expressed in mL/min/mmHg. This test helps evaluate conditions that affect the gas exchange surface, such as emphysema, fibrosis, or vascular diseases.

Computed tomography is an imaging method that shows anatomy, not gas transfer across the alveolar–capillary membrane. Diffusion capacity testing is not a measure of lung volumes, and it is not a sleep study or a test of airway resistance during sleep.