Carbon monoxide transfer coefficient (transfer factor/alveolar volume) in females versus males.

The single breath transfer factor of the lung for carbon monoxide (TL,CO) is derived from the multiple of the carbon monoxide transfer coefficient (KCO) and the alveolar volume (VA) during breath holding [1]; consequently, the TL,CO is highly dependent on lung size (yVA), and thus is smaller in females for a given height and age. Essentially, the KCO is the rate constant for alveolar carbon monoxide uptake (its units are per second or per minute) and may be expected to be the same in males as in females, as it was in the original description of the technique [2]. The published guidelines [3, 4] are not definitive on this point, so we reviewed all studies of reference values for TL,CO and KCO, which have presented data for both males and females. The European reference values for KCO [3] are derived from the ratio TL,CO predicted/total lung capacity (TLC) predicted. This unusual recommendation arose from correspondence in the ERJ [5], which pointed out that the reference equations previously published in 1983 [6] produced values for KCO that were incompatible with those for TL,CO. These European Respiratory Society reference values have been quoted in 118 articles published since 1993 (23 in the ERJ) and have been adopted elsewhere [7]. The TL,CO values for females were based on a summary equation [6] derived from only five small studies, the most recent being published in 1979. Fortunately, the available database for TL,CO and KCO has now expanded considerably; seven studies that examined both males and females in their chosen populations have been published between 1980 and 1992 (table 1). In addition, we believe the use of the TL,CO pred/TLC pred ratio is, itself, unjustified. First, because in general it is undesirable to ratio two reference values obtained from different populations; this is compounded in the present instance, when KCO is inevitably measured in every individual in the actual manoeuvre used to obtain TL,CO. Secondly, although the original recommendation [18] was that the lung volume during breath-holding (yVA) should be measured from the inspired volume preceding the breath hold plus the residual volume (from closed circuit helium dilution), virtually all subsequent population studies (and all those in table 1) have substituted the VA derived from the dilution of the inhaled marker gas. This single breath VA, both in the derivation of TL,CO and in the subsequent calculation of TL/VA, is always less than TLC, because …