1. 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).

2. 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.

3. The blood clearance of IgG-RBC was mono-exponential, whereas that of HD-RBC was bi-exponential. 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 > 0.05 at high coating; r = 0.75, P < 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 < 0.001 at high coating; r = 0.76, P < 0.05 at low coating) but significantly closer only at high coating.

4. 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<0.001).

5. By using splenic blood flow based either on platelets or on the initial slope of the HD-RBC clearance, an estimate was made of splenic IgG-RBC extraction ratio. The normal extraction ratio at high coating was about 50% and at low coating about 30%. Some patients with systemic lupus erythematosus and rheumatoid arthritis appeared to have abnormally low extraction ratios.

6. It was concluded that HD-RBC and IgG-RBC are not interchangeable as markers of splenic reticuloendothelial function; HD-RBC clearance is mainly dependent on splenic blood flow whereas IgG-RBC clearance is dependent both on splenic blood flow and splenic macrophage function. The simultaneous use of the two markers allows therefore an estimate of the splenic IgG-RBC extraction ratio, which potentially is a more specific index of splenic reticuloendothelial function than the clearance rate of either marker used alone.

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