Expansion
Measured with carbon monoxide, reflecting surface area, membrane thickness and haemoglobin
Expansion
TLCO (transfer factor) measures the uptake of a trace of carbon monoxide in a single breath. KCO is TLCO corrected for alveolar volume.
Carbon monoxide is ideal because its avid haemoglobin binding keeps its capillary partial pressure near zero, so uptake is limited entirely by diffusion, not by perfusion.
Reduced
- Emphysema (loss of surface area)
- Pulmonary fibrosis (thickened membrane and lost area)
- Pulmonary embolism and pulmonary hypertension (reduced capillary blood volume)
- Anaemia, which must be corrected for
Increased
- Pulmonary haemorrhage, since blood in the alveoli takes up carbon monoxide
- Polycythaemia
- Asthma, left to right shunts and exercise
The pattern is diagnostically useful: obstruction with a low transfer factor suggests emphysema, whereas obstruction with a normal transfer factor suggests asthma.