TRANSFER FACTOR ANALYSER

Typical Number in Hospital: 1

Cost Bands: 5

References: 3

When a person is unable to breathe properly, oxygen does not enter the blood in sufficient quantities. The performance of different elements in the respiration mechanism may be measured separately (such as vital capacity or residual volume), but overall performance can best be described by the transfer factor which quotes the quantity of carbon monoxide which enters the blood each minute for every mmHg of partial pressure of the gas.

Carbon monoxide is used because it is readily absorbed by the haemoglobin whereas oxygen passes in both directions and would therefore be difficult to quantify. A typical procedure might be for the patient to breathe in a measured volume of gas containing known concentrations of carbon monoxide and helium. The breath is then held for ten seconds or so to allow mixing and transfer to take place. The concentrations of the two gases are then measured in the exhaled gases. The total volume of gases in the system can then be estimated from the new concentration of the helium, which enables calculation of the expected concentration of carbon monoxide if none had been transferred to the blood. The difference between this estimated concentration and the measured concentration is the quantity absorbed.

The small concentration of carbon monoxide used (less than 1% because of its high toxicity) is measured in an infrared analyser. Carbon monoxide and dioxide gases absorb infrared light at different characteristic wavelengths and so the analyser can be specific to one or other gas. The helium analyser would be the same as described for the Helium FRC analyser.

Transfer factor analysers are found in the lung function laboratory.