THE REPRODUCIBILITY AND ACCURACY OF PLASMA VOLUME ESTIMATION IN THE SHEEP WITH BOTH 131 I GAMMA GLOBULIN AND EVAN'S BLUE.

Summary The reproducibility and accuracy of estimation of plasma volume in the sheep were investigated for each of two separate plasma labels— 131 I gamma globulin and Evan's blue dye. The results showed that the 131 I gamma globulin space was more reproducible than the simultaneously estimated Evan's blue space (P < 0·05), and that the latter indicator measured a space some 12% larger than the former (P < 0·01). The “error” term associated with the estimation of the 131 I gamma globulin space in well‐trained animals (coefficient of variation 2·5%) was comparable with the theoretical error (coefficient) of variation 1·2%) derived by combining the errors associated with the individual methodological procedures. In less well‐trained animals the in vivo “error” for the estimation of the 131 I gamma globulin space was significantly higher than that observed in the well‐trained animals (P < 0·01). It is suggested that this may have been due to a variation in the amount of circulating protein and ultrafiltrate during the experiments in the less well‐trained animals. Evidence for the accuracy of estimation of true plasma volume with 131 I gamma globulin is discussed, with particular reference to the finding of a high “F cells” ratio (mean 0·96) In a chronically splenectomised sheep, using 131 I gamma globulin and 31 Cr labelled red cells.