Which statement about DLCO testing is correct?

• A. It measures airway resistance during forced expiratory maneuvers.
• B. It assesses diffusion capacity for carbon monoxide across the alveolar-capillary membrane.
• C. It evaluates pulmonary venous pressure.
• D. It quantifies dead space ventilation.

Answer: (B)

DLCO testing measures how readily gas diffuses from the air spaces into the blood across the alveolar-capillary membrane. Carbon monoxide is used because it binds to hemoglobin very readily, so the rate of uptake reflects the diffusion process and the capillary blood volume, not ventilation or airway resistance. The test is usually performed with a single-breath technique: the patient exhales to residual volume, inhales a small amount of gas containing CO, holds briefly, then exhales; the drop in CO in the exhaled air is used to calculate the diffusion capacity, with adjustments for hemoglobin level and lung volume. This makes DLCO sensitive to conditions that thicken or thin the diffusion barrier or alter capillary blood volume, such as interstitial lung disease, fibrosis, emphysema, or pulmonary vascular disease.

Other statements don’t describe what DLCO measures: measuring airway resistance relates to airflow mechanics and is obtained through spirometry/plethysmography, not DLCO; assessing pulmonary venous pressure is a hemodynamic parameter, not a diffusion test; and dead space ventilation concerns ventilation without perfusion, which is not what DLCO evaluates.