Serum-induced platelet procoagulant activity: An assay for the characterization of prothrombotic disorders

Platelets contribute to hemostasis by forming a platelet plug and by provid ing a procoagulant surface for the assembly and activation of the coagulati on factors. The contribution of platelets to prothrombotic disorders has be en difficult to analyze. Recently an assay was reported that measured the p rocoagulant activity of test platelets by making the platelet lipid surface the limiting factor in the production of thrombin, In this report we descr ibe a novel technique, based on this assay, that we used to study patient s erum factors that activate control platelets and in turn initiate measurabl e procoagulant activity. Using this assay we investigated a group of patien ts with prothrombotic disorders. The patient test serum was incubated with normal platelets in the presence of activated factor Xa. The resultant thro mbin was measured in a chromogenic assay. The rate-limiting step was the pr esence of any potential platelet-activating factors, such as antibodies in the heat-treated test serum, that would allow the Xa to bind to the platele t phospholipid surface. Serum samples from patients with heparin-induced th rombocytopenia (HIT) and the anti-phospholipid antibody syndrome enhanced p latelet procoagulant activity, while samples from patients with idiopathic thrombocytopenic purpura and disseminated intravascular coagulation (DIC) d id not. HIT serum samples also induced platelet activation, as measured by platelet microparticle shedding, carbon 14-labeled serotonin release, and p latelet aggregation. The measurement of serum-induced platelet procoagulant activity provides a method for the investigation of circulating platelet a gonists in prothrombotic disorders.