Blood Volumes of Dairy Calves Comparing Cr⁵¹-Tagged Red Blood Cells and T-1824 Plasma Dilution Methods

¹ From the Departments of Dairy Husbandry and Veterinary Physiology, Division of Agricultural Science, University of Missouri, Columbia, Missouri

In a repeated study on 17 dairy calves, T-1824 dye plasma dilution showed significantly higher blood volumes than were found by any other technique or computation method using Cr⁵¹-tagged red blood cells. Five blood samples taken at 20-minute intervals after injection showed consistent decrease in radioactivity count from the first to the last sample, indicating greater accuracy in radioactivity dilution regressed to zero time figures than in average counts of several postinjection samples. In vitro studies suggest a loss of Cr⁵¹ from red blood cells to plasma after saline washings are Cr-free. Percentage blood volumes computed from whole blood samples of calves injected with Cr⁵¹-tagged red blood cells decreased in a straight line relationship with increase of body weight. Percentage plasma and whole blood volumes estimated with the T-1824 dye technique decreased regularly with body weight increase until a second determination was made when there was a rapid rise nearly to the level of the smallest calves, followed by another regular decrease with increase in weight. It is suggested that repeated dye injections do not always measure the same space. Regressed values of five whole blood samples taken at 20-minute intervals after injection of Cr⁵¹ tagged red blood cells gave more consistent blood volume determinations than either the weighed red cells or the plasma dye dilutions of the same samples.