Which test assesses the lungs' diffusing capacity for carbon monoxide (DLCO)?

• A. DLCO
• B. Polysomnography
• C. Bronchoscopy
• D. L/S ratio

Answer: (A)

The main idea is how we evaluate the lung’s ability to transfer gas from the air into the blood. The test that does this is a DLCO test, which directly measures the diffusing capacity of the lungs for carbon monoxide. Carbon monoxide is used because it binds hemoglobin very quickly and its uptake is limited by diffusion across the alveolar–capillary membrane, making DLCO a sensitive indicator of the transfer process.

In practice, the test usually involves inhaling a small amount of a CO-containing gas, holding the breath briefly, then exhaling and analyzing the gas to calculate how much CO moved from the lungs into the blood. The result reflects the surface area of the alveolar-capillary membrane, the pulmonary capillary blood volume, and the thickness of the barrier. A reduced DLCO points to diffusion impairment from conditions like emphysema, interstitial lung disease, or pulmonary edema; a normal or high value helps rule out diffusion limitation as a cause of symptoms.

The other tests measure different aspects of respiratory physiology. Polysomnography monitors sleep stages and events to diagnose sleep disorders. Bronchoscopy involves direct visualization or sampling of the airways. The L/S ratio assesses fetal lung maturity, not the diffusion capacity of an adult patient’s lungs.