Expansion
TLCO measures diffusion; KCO corrects it for the volume available
Expansion
What it measures: the uptake of a small dose of carbon monoxide, which is limited by diffusion, so it reflects the alveolar-capillary membrane, the surface area available and the haemoglobin in the capillaries.
- TLCO (DLCO): total transfer of the lung
- KCO: TLCO divided by the alveolar volume, so transfer per unit of accessible lung
Reduced transfer factor
- Parenchymal: interstitial lung disease, emphysema, pneumonia, oedema
- Vascular: pulmonary embolism, pulmonary hypertension, vasculitis
- Loss of lung: pneumonectomy, lobectomy
- Anaemia, which reduces the haemoglobin available to bind carbon monoxide, so the result is corrected for haemoglobin
- Smoking, which raises baseline carboxyhaemoglobin
Raised transfer factor
- Alveolar haemorrhage, classically Goodpasture’s syndrome, where free blood in the alveoli takes up carbon monoxide avidly
- Polycythaemia
- Asthma, and left to right shunt
- Exercise immediately before the test, and the supine position
The clinically useful distinction
- Restriction with a low transfer factor: intrapulmonary, so fibrosis or infiltration
- Restriction with a normal or high KCO: extrapulmonary, so chest wall disease, neuromuscular weakness or obesity, where the lung tissue itself is normal but underinflated
- Obstruction with a low transfer factor: emphysema
- Obstruction with a normal or high transfer factor: asthma or chronic bronchitis
This last pair is the single most useful application of the test in practice.