Determination of plasma volume by 113min-labeled transferrin

The binding property of ionic indium (In3+) with plasma transferrin was utilized for determination of plasma volumes (PV) of whole body and individual organs in small animals. Plasma transferrin from a donor rat was labeled with 15-17 muCi 113mIn/ml plasma and injected into the tested rats. PV were determined either by extrapolation to the dilution at time zero (for whole animals) or by calculation of the ratio, organ radiation: radiation of a plasma unit volume (for organs). The reliability of the method for determination of whole-body PV was ascertained by comparing the results obtained with those obtained simultaneously by the Evans blue dilution method. Whole body PV values obtained by the two methods were similar, with a correlation coefficient (r) of 0.997. The short half life of 113mIn enables it to be used with other nuclides which have similar or different energies in the same sample; indium radiation was counted first and after it had disappeared the activity of the other nuclide could be measured.