Factor VIII and Ristocetin-Induced Aggregation in sickle Cell Disease

Haemostatic variables were studied in 23 patients with sickle cell disease (SCD) who were followed over a year. Investigations included measurements of factor VIII_C, VIII_Ag, antithrombin III biological and antigen levels, fibrinogen and platelet aggregation to ADP, collagen, porcine AHG and ristocetin. All eight SS patients in the steady state had normal levels of F VII_C and high levels of F VIII_Ag with marked discordance between the two values (mean VIII_c: VIII_Ag ratio 0.3). By contrast, 15 patients with 15 disease or S thalassaemia tended to show less Barked discordance between levels of F VIII_C and F VIII_Ag (mean VIII_c VIII_Ag 0.7). Patients in the steady state showed normal platelet aggregation to ADP, collagen and porcine AHG, but ristocetin induced aggregation was absent or œarkedly reduced. Platelet aggregation to ristocetin is absent in most individuals of Afro-Carribean origin and the ristocetin co-factor activity may be difficult to assess in this ethnic group.

During crisis both factor VIII_c and factor VIII_Ag were invariably high. Factor VIII_Ag was studied using crossed Immunoelectrophoresis in 5 patients. In one patient two peaks were found and in the others the F VIII_Ag had more anodal mobility than normal. These findings are suggestive of the presence of lower molecular weight fragments of factor VIII_Ag and may account for the apparent very high levels as measured by Laurell rocket technique.