7th February 2024

Diffusing capacity or transfer factor of the lung

The diffusing capacity of the lung for carbon monoxide (DLCO) is a measurement of the lung’s ability to transfer gases. It is alternatively called the transfer factor of the lung (TLCO) – and uses different units.

Carbon monoxide is the only gas used for this test as it is diffusion-limited. Here is why. Carbon monoxide has a 250 times greater affinity for Hb than oxygen. Hence, CO is rapidly bound to Hb, resulting in low plasma partial pressure of CO. There will still be a partial pressure difference between alveolar CO and plasma CO at the pulmonary end of capillary blood.

DLCO or TLCO is commonly done during pulmonary function tests. It is done by measuring the partial pressure of CO-inspired and expired. The difference between inspired and expired CO is the amount of CO that has diffused across the alveolar-capillary barrier.

Causes that decrease DLCO:

• Pulmonary fibrosis
• Pneumonia
• Pulmonary hypertension
• Chronic pulmonary emboli
• Emphysema
• Congestive cardiac failure

Causes that increase DLCO:

• Asthma
• Left-to-right shunts
• Pulmonary haemorrhage

Note: Haemoglobin count (anaemia/polycythaemia) affects DLCO. Hence, DLCO is adjusted for Hb values.