Influence of the Amount of Platelet Bound IgG (Antibody) on Platelet Survival and Site of Sequestration in Autoimmune Thrombocytopenia

This study aims to determine if increasing amounts of antiplatelet antibody result in progressive shortening of platelet survival and in proportionately greater hepatic sequestration. Antiplatelet antibody was measured by a quantitative antiglobulin technique using complement mediated lysis of IgG sensitised sheep RBC as an indicator system (1). Platelet survival was measured with autologous or ABO Rh compatible isologous platelets labelled with Cr-51 according to ICSH specifications (2). The survival data was fitted to linear exponential, multiple exponential and gamma functions on a Univac 1106 computer, incorporating a X² measure of goodness-of-fit. Appropriately collimated detectors over liver and spleen were used to measure the relative uptake of labelled platelets which was expressed as the liver:spleen ratio at 48hr. after injection of the dose.

In 15 untreated patients with ITP, platelet antibody was 21-213ng IgG/10⁶ platelets (normal 1.8 - 14.8, n=47). Survival data was best described by 2 exponentials. No correlation was observed between platelet antibody levels and the 2 exponential estimate of platelet meanlifespan. A highly significant correlation (p<0.001) was obtained between platelet antibody and the liver:spleen ratio demonstrating that greater amounts of antibody are associated with progressively greater hepatic sequestration.

(1) Dixon et al, New Engl.J. Med.292; 230. (2) Blood (1977) 50; 1137.

Supported by the Nellie Atkinson Bequest.