The Reproducibility and Accuracy of Plasma Volume Estimation in the Sheep With Both 131i 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—131I gamma globulin and Evan's blue dye. The results showed that the 131I 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 131I 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 131I 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 131I 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 131I gamma globulin and 31Cr labelled red cells.