A new method for measuring the blood volume of the rat using 113mIndium as a tracer

A new method is described for the estimation of blood volume in the rat using 113mIndium chloride. No in vitro labelling step is necessary as the isotope binds specifically to transferrin in vivo. Under anaesthesia, an external jugular vein was cannulated for blood sampling and indium chloride injected directly into a lateral tail vein. Three blood samples of 0.2 ml were taken at short intervals and radioactivity measured under standard conditions. Blood volume was found to be 7.46 +/- 0.14 (mean +/- SEM) ml/100 g body weight. Repeating the study of 5 animals gave similar values for blood volume on the 2 occasions (7.23 +/- 0.26 and 6.95 +/- 0.23 ml/100 g, P > 0.05). The new technique was compared with established methods using 51Cr labelled red blood cells and 125Iodinated albumin. In each animal, the 113mIndium technique produced values approximately 10% higher than those obtained using the sum of plasma and red cell mass, in keeping with the known difference between whole body and large vein haematocrit (Indium, 7.20 +/- 0.19 and Iodine with Chromium, 6.40 +/- 0.34 ml/100 g). Data on blood volume determined using 125Iodine as the tracer on these same animals were identical (Indium, 7.20 +/- 0.19 and iodine, 7.16 +/- 0.34 ml/100 g). The method is simple to perform and appears to be at least as accurate and reproducible as established methods for measuring blood volume.