Diffusing Capacity of the Lungs for Carbon Monoxide (DLCO)

Question 1

What does DLCO measure?

Answer 1

It measures how well gases (especially oxygen) pass from the alveoli into the pulmonary capillary blood — essentially, the efficiency of gas exchange at the alveolar-capillary membrane

Question 2

🔬 How is DLCO measured?

Answer 2

The patient inhales a small, harmless amount of carbon monoxide (CO).

CO is used because it binds to hemoglobin similarly to O₂ but is not normally present in blood, making it ideal for measurement.

The amount exhaled vs inhaled reflects how much CO diffused across the alveolar membrane.

Question 3

What does a low DLCO suggest

Answer 3

Low DLCO suggests:
Impaired alveolar-capillary membrane or reduced pulmonary blood flow

Examples:
- Pulmonary fibrosis 🫀 (thickened membrane)

• Emphysema (destruction of alveolar walls)
• Pulmonary embolism (reduced perfusion)
• Anemia (less hemoglobin to bind CO)

Question 4

What does a high DLCO (less common) suggest

Answer 4

High DLCO (less common) suggests:

• Asthma (increased blood volume in capillaries)
• Polycythemia (more hemoglobin)
• Alveolar hemorrhage