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A child was seen after an acute illness with bruising, the “lupus anticoagulant”, and the virtual absence of plasma prothrombin clotting activity. Screening test results were: patient’s plasma, PTT 117", prothrombin time 36"; 1/1 mixture of patient’s and normal plasma, PTT 162" (cofactor phenomenon), prothrombin time 18". F. XII, XI, IX, and VIII assays were typical for the “lupus anticoagulant”, yielding higher values with increasing dilution of the patient’s plasma. The patient’s plasma contained 0.05 U/ml of prothrombin activiy in a one-stage assay and no demonstrable prothrombin antigen by double immunodiffusion or electroimmunoassay with a goat anti-prothrombin anitserum. Prothrombin clotting activity was not inhibited when the patient’s plasma was incubated with varying concentrations of normal plasma or purified human prothrombin. Nevertheless, when the patient’s plasma was used as a source of antibody in agar immunodiffusion plates precipitin lines of identity were observed against normal plasma and purified prothrombin. Presence of an antibody against human prothrombin was further demonstrated by incubating varying concentrations of 125I-prothrombin with the patient’s plasma followed by ammonium sulfate precipitation of the antigen-antibody complex (estimated affinity, 2.5 × 109). Over a 3-week period prothrombin activity and antigen increased, prothrombin antibody decreased, and the bruising subsided. This study demonstrates: (1) that the acquired deficiency of prothrombin clotting activity in this rare syndrome stems from the disappearance of the prothrombin molecule from the plasma, and (2) that a prothrombin antibody is present in the plasma despite its inability to neutralize prothrombin in vitro. Rapid clearance of prothrombin antigen-antibody complexes could be a mechanism for the acquired hypoprothrombinemia of this unusual syndrome.

thrombosis and haemostasis

Syndrome Of Acquired Prothrombin Deficiency And The “Lupus Anticoagulant”: Demonstration Of A Prothrombin Antibody That Does Not Neutralize Prothrombin Clotting Activity In Vitro