Simultaneous distribution of T-1824 and I131 labelled human serum albumin in man.

The dilution of injected dyes (1-3), especially Evans Blue (T-1824) (4), has gained wide acceptance as a measure of plasma volume (5, 6). Recently, such an interpretation of the "T-1824 space" has been questioned. The challenges are based largely upon discordant volumes of distribution obtained after the simultaneous administration of T-1824 and labelled red blood cells (7-14), or T-1824 and radioactive plasma proteins (15-18). Although others have been unable to confirm these differences (19-22), the reported divergences are in agreement, both in direction and magnitude. In every instance T-1824 was diluted to a greater extent than its companion indicator, and the ratios of T-1824 space/Indicator X space exceeded unity by 10 to 20 per cent. The intravascular retention of T-1824 depends upon the combination of free dye anions with serum albumin (23). Two hypotheses have been advanced to explain the discrepant results obtained with T-1824 and other indicators. First, there might be early abstraction of T-1824 by phagocytosis or staining of tissues (15, 24, 25). Secondly, a disproportionate loss of T-1824 might occur during mixing by penetration into erythrocytepoor (7-14, 25, 28) or even extravascular channels (16, 17, 25-29). The first explanation rejects T-1824 as an adequate protein label; the second explanation questions the immediate distribution of the T-1824-albumin complex. Evidence points to the formation of a stable bond between T-1824 and albumin in vitro (30-33).