DLCO, or Diffusing Capacity of the Lung for Carbon Monoxide, is a specialised pulmonary function test that measures the lungs’ ability to transfer oxygen from the air into the bloodstream. It evaluates the efficiency of the alveolar-capillary membrane, which is critical for oxygenation. DLCO testing is essential in diagnosing, monitoring, and managing various lung diseases, including interstitial lung disease, chronic obstructive pulmonary disease (COPD), pulmonary fibrosis, emphysema, pulmonary hypertension, and anaemia-related hypoxia.

During the test, the patient inhales a small, harmless amount of carbon monoxide mixed with a tracer gas and holds their breath for a few seconds before exhaling. The concentration of carbon monoxide in the exhaled air is measured to calculate how effectively oxygen would diffuse across the lung membrane. The procedure is non-invasive, safe, and usually takes 10–15 minutes to complete.

DLCO testing is particularly useful when standard lung function tests, such as spirometry, are normal but the patient continues to experience unexplained shortness of breath or fatigue. It provides early detection of subtle lung impairments and helps differentiate between obstructive and restrictive lung diseases. Serial DLCO measurements are also used to monitor disease progression and response to therapy in chronic lung conditions.

The procedure is performed by trained respiratory therapists in a controlled pulmonary function laboratory. Patients are advised to avoid smoking or vigorous exercise before the test to ensure accuracy. Results are interpreted by pulmonologists in the context of clinical evaluation, imaging, and other pulmonary tests.

By providing a precise assessment of lung gas exchange, DLCO is a vital tool in respiratory medicine. It guides diagnosis, treatment planning, and monitoring, ultimately helping patients maintain optimal lung function, manage chronic conditions effectively, and improve overall respiratory health.