To the Editor:
In their recent “Perspective”, Hughes and Pride 1 described the carbon monoxide (CO) transfer factor (TL,CO) as the product of two primary measurements: alveolar volume (VA) and CO transfer coefficient KCO (TL/ VA). VA is primary in that it has a physiological identity. However, Forster 2 has pointed out that in the context of the measurement of transfer factor, the volume is merely that of the gas exchanger; it has no more physiological significance than can be attached to the volume (VA+Vbag) that is used for the rebreathing method. Forster 2 also warned against interpreting KCO as a separate index. Its status as a primary index would have been different if Krogh 3 had been right in her belief …