A sensitive and specific test was used to identify a platelet-agglutinating factor in sera from patients with thrombotic thrombocytopenic purpura. Serum from patients plus a preparation rich in large multimers of factor VIII: von Willebrand factor were added to target platelets, and agglutination occurred in 41 of 48 samples. Edetic acid, heparin, or heating, but not aspirin, monomeric IgG, or dansylarginine N-(3-ethyl-1,5-pentanediyl)amide inhibited the platelet-agglutinating factor. In-vitro agglutination requires the presence of a platelet-agglutinating factor and large multimers of von Willebrand factor. High concentrations of either component lowers the amount of the other required for platelet agglutination. Some patients may be more susceptible to the agglutinating factor because of a congenital or acquired abnormality in processing unusually large multimers of von Willebrand factor or because of infections or inflammatory disorders that lead to increased synthesis of large multimers of von Willebrand factor.
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