Sodium pertechnetate as a red cell label: in vitro and in vivo studies

In vitro investigations were performed to re-examine the variables that influence the binding of sodium 99TCm-pertechnetate to human red blood cells. Irreversible binding of the radioisotope was observed only when red cells were incubated with tin chloride after their incubation with pertechnetate; binding of pertechnetate is completed within 10 min after its addition to red cells. Composition, temperature and possibly the oxygen saturation of the incubation medium affect the fraction of the isotope that becomes red cell-bound. Using a labelling method derived from these in vitro studies, the usefulness of pertechnetate-labelled autologous red cells for the determination of red cell volumes was studied. Elution of pertechnetate occurred in vivo in an exponential fashion. Using the specific red cell activity, corrected for elution, of blood samples drawn 30 min after injection of 99Tcm-labelled red cells, calculated red cell volumes were almost identical to those determined simultaneously with radiochromate-labelled cells. It is concluded that 99Tcm-pertechnetate is a useful label for red cell volume determination; it may be less reliable when delayed mixing of the labelled red cell tracer in the patient's circulation is expected.