Plasma exchange and plasma infusion are effective in the treatment of patients with thrombotic thrombocytopenic purpura (TTP). It has been proposed that some component in normal plasma may be deficient in TTP patients. In this experiment normal plasma was fractionated and an effective fraction for the treatment of TTP was sought. Plasma was fractionated with 5% polyethylene glycol and a large molecule fraction was obtained which was composed mainly of proteins and lipoproteins with molecules larger than that of IgG. The fraction was dissolved in physiological saline and infused in two patients with chronic relapsing TTP who had been successfully treated periodically with plasma infusion. Peripheral platelet counts and serum LDH levels were measured, and the efficacy of the treatment was assessed. Infusion of the large molecule fraction elevated platelet counts and lowered the LDH level. However, the efficacy of the large molecule fraction was 25 to 50% that of plasma. Components in the fraction may have been inactivated partially during preparation. We propose that the large molecule fraction contains factors which are effective in some patients with TTP. © 1995 Wiley‐Liss, inc.
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