A 66-year-old man with peptic ulcer disease developed a paraprotein that resulted in a spontaneously prolonged prothrombin time, activated partial thromboplastin time, and thrombin clotting time. Although the reptilase time was normal, the thrombin clotting time failed to correct with the addition of normal plasma, calcium, or protamine sulfate. The patient's purified fibrinogen was normal, but his serum contained an IgG that inhibited the clotting of normal plasma and purified fibrinogen in the presence of thrombin. In contrast to previously described paraproteins, this patient's IgG appeared to inhibit the activity of thrombin per se rather than to interfere with fibrinogen cleavage or fibrin polymerization. Although immunoprecipitation between thrombin and the paraprotein could not be demonstrated, the patient's purified IgG, in the presence of thrombin, decreased the thrombin activity on a chromogenic substrate. Further, increasing concentrations of thrombin overcame the inhibitory effect of the patient's paraprotein. Thus, the patient's paraprotein appeared to possess antithrombin activity.
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