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The transfer coefficient is the value of the transfer factor divided by the alveolar volume. This value is an expression of the gas transfer ability per unit volume of lung.

The test is performed as described for the transfer factor; in addition the inhaled gas contains 10% helium. Because helium is not absorbed, the dilution of the helium in the exhaled air permits the calculation of the alveolar volume.

The normal values for KCO are dependent on age and sex. A fit young adult may have a KCO of approximately 1.75 mmol/min/kPa/litre, an elderly adult may be about 1.25.

This parameter is useful in the interpretation of a reduced transfer factor. For example, if the patient has a disease that causes a decrease in lung surface area, or has had a lung removed, then there is a decrease in transfer factor but there is a normal KCO. However, in conditions such as fibrosing alveolitis or emphysema, where there is damage to the lung parenchyma there is a reduction in both transfer factor and transfer coefficient. On a similar note, if a reduction in lung volume is due to an inability to expand the thorax (e.g. weakness) then the TLCO is low but the KCO is normal or increased.

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