Blood Volume in the Rat

The organ distribution of radiopharmaceuticals in the rat is usually estimated using 7% of body weight (BW) for blood volume (BV). In spite of its important impact on the evaluation of new agents, this value has not been validated adequately. We therefore studied blood volume in 70 awake Wistar rats (100 to 400 g BW) in which red blood cell volume (RBCV) and plasma volume (PV) were measured simultaneously. Red blood cell volume was measured by in vitro RBC-tagging with Tc-99m in Sn-pyrophosphate, 0.05 microgram per ml of blood; plasma volume was measured with I-125 human serum albumin (HSA). Ten minutes after injection of the dose, 0.5 ml of blood was withdrawn from the carotid or femoral artery and duplicate samples of 0.025 ml of blood were counted after separating RBCs from plasma. Total blood volume was calculated by adding RBC volume and plasma volume. The relationship for the entire group was: BV (ml) = 0.06 X BW + 0.77 (r = 0.99, n = 70, p less than 0.001). The difference between male and female rats was not statistically significant. The use of an arbitrary value of 7% for estimation of blood volume can lead to significant errors in calculating radiopharmaceutical distribution. The use of the general formula for the blood-volume calculation described here should improve the accuracy and reliability of estimates of radiopharmaceutical distribution.