The comparative blood clearance kinetics of modified radiolabelled erythrocytes.

The rate of clearance from blood of 111In-labelled heat damaged autologous erythrocytes (HD-RBC) has been compared with that of simultaneously injected autologous 99mTc-labelled erythrocytes (IgG-RBC) coated with a Rhesus anti-D antibody. In 17 studies, the number of antibody molecules coating the erythrocytes was 9000 (high coating) and in nine studies the number was 5000 (low coating). On gamma camera imaging, IgG-RBC uptake, at both levels of coating, could be visualized only in the spleen. HD-RBC were predominantly taken up by the spleen, although slight 111In activity was visible in the liver. The blood clearance of IgG-RBC was monoexponential, whereas that of HD-RBC was biexponential. The reciprocal of the t1/2 (the time taken for the 3 min value to fall by 50%) of the HD-RBC clearance correlated rather poorly with the rate constant of the simultaneous IgG-RBC clearance (r = 0.47, P greater than 0.05 at high coating; r = 0.75, P less than 0.05 at low coating). The rate constant of the second exponential of the HD-RBC clearance showed a correlation with the rate constant of IgG-RBC clearance that was closer than the reciprocal of the t1/2 of HD-RBC clearance (r = 0.89, P less than 0.001 at high coating; r = 0.76, P less than 0.05 at low coating) but significantly closer only at high coating. Splenic blood flow, measured using indium labelled platelets in ten subjects, correlated closely with the initial slope of HD-RBC clearance (r = 0.93, P less than 0.001).