ACTIVATED PLATELET AGGREGATES IN THROMBOTIC THROMBOCYTOPENIC PURPURA - DECREASE WITH PLASMA INFUSIONS AND NORMALIZATION IN REMISSION

Circulating activated platelet aggregates (aPA) were assayed by now cy tometry employing mAb alpha-CD62p in eight patients with thrombotic th rombocytopenic purpura (TTP). Elevation of aPA was observed in all pat ients in active stages of TTP; aPA normalized in remission. Plasma inf usions with plasmapheresis decreased aPA in responding patients. The r ise and fall of aPA preceded relapses and improvements, respectively. These changes were seen prior to the traditional indicators. LDH, haem atocrit, and platelet count. Incubation of plasma from TTP patients wi th normal whole blood induced formation of aPA; this effect was signif icantly greater than that of plasmas from ITP patient controls (P < 0 . 01), suggesting the presence of an aPA-promoting factor in TTP plasm a. Parallel experiments using a platelet aggregometer failed to detect effects of TTP plasma on normal blood. In summary, aPA appear to be a marker of disease activity, rising with relapse, falling with plasma therapy, and normalizing in remission. The flow cytometric assay of aP A is more sensitive than aggregometry in detecting the putative aPA-pr omoting factor in TTP.