The splenic extraction ratio of antibody-coated erythrocytes and its response to plasma exchange and pulse methylprednisolone.

Splenic blood flow was measured in a series of normal subjects and patients with connective tissue diseases by measuring the rate of equilibration and the partition of 111In-labelled autologous platelets between blood and spleen. These data were used to quantify the role of splenic blood flow in determining the splenic clearance of IgG coated erythrocytes (IgG-RBC) from the circulation. Previous studies have interpreted the clearance rates of IgG-RBC only in terms of splenic reticuloendothelial function. Splenic blood flow was increased in seven of eight patients with systemic lupus erythematosus (SLE), six of 10 patients with rheumatoid arthritis (RA) and in all five patients with essential mixed cryoglobulinaemia (EMC) compared with a series of thirteen normal subjects. Expressing the rate constant of clearance of IgG-RBC as a fraction of splenic blood flow gave a value for the 'extraction ratio' of IgG-RBC (a specific measurement of reticuloendothelial function, corrected for splenic blood flow). Normal splenic extraction ratio of IgG-RBC was calculated to be 32%. All the patients with SLE and with EMC had reduced extraction ratios (in seven out of 13 patients less than 10%). In RA the extraction ratio tended to be normal (average 27.3%) but variable (9-59%). Following plasma exchange in nine patients, a significant increase in IgG-RBC extraction ratio (average of 39% with respect to pre-exchange values, P less than 0.05) was found. In contrast there was no significant change in extraction ratio following pulse methylprednisolone therapy in a further nine patients. Although the rate constant of clearance of IgG-RBC decreased by an average of 33% (P less than 0.01) in the latter group, it was matched by an equal decrease of splenic blood flow (average 37%, P less than 0.01) and so extraction ratio showed no change. These data indicate that quantification of splenic reticuloendothelial function requires measurement of both IgG-RBC clearance and of splenic blood flow.