A single measurement with 51Cr-tagged red cells or 125I-labeled human serum albumin in the prediction of fractional and whole blood volumes: an assessment of the limitations

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This study investigated variations in the prediction of blood volumes from a single measurement of red cell volume (RCV) with 51Cr or plasma volume (PV) with 125I human serum albumin (HSA). In 111 subjects, fractional and whole blood volumes were estimated from separate direct measurements of RCV and PV. The f ratio (body to venous hematocrit) was also determined. There was a very good correlation between 125I-HSA measured PV (2857 ± 822 ml) and that estimated with 51Cr-tagged red blood cells (2864 ± 747 ml) (r = 0.936, p = 0.000) and also between 51Cr measured RCV (2600 ± 774 ml) and that estimated with 125I-HSA (2589 ± 843 ml) (r = 0.944, p = 0.000). The 95% limits of agreement (mean ± 2SD of differences, relative to the mean of paired data) ranged −0.2% ± 20.3% and 0.4% ± 21.4%, respectively. The 95% prediction intervals of measured from estimated fractional blood volumes spanned ±20.3% and ±19.5%, respectively, relative to the predicted values with regression equations. Proportional degrees of inaccuracy were found in whole blood volume estimations. The f ratio was inconstant and correlated with PV and the body hematocrit. We conclude that blood volumes can be determined reliabily only with direct measurements of RCV and PV. Estimated blood volumes may lead to misconceptions.