A patient with a significant history of spontaneous and posttraumatic bleeding was found to have hypoprothrombinemia. His prothrombin (factor II) activity by clotting assay was 9.5% and his factor II antigen was 5%. Crossed immunoelectrophoresis and sucrose density gradient ultracentrifugation of the patient's plasma showed his prothrombin to be qualitatively indistinguishable from normal prothrombin by these techniques.

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