Mechanism: Antibodies bind to prothrombin without neutralizing its in vitro coagulant activity. The prothrombin-antibody complexes are removed in vivo, resulting in depletion of prothrombin levels.
Features:
(1) presence of a lupus anticoagulant
(2) either (a) prothrombin time in a non-anticoagulated patient > 3 seconds above upper limit of normal reference range, or (b) prothrombin time prolonged greater than expected for warfarin dose or warfarin dosing difficult to control, or (c) prothrombin time prolonged in a patient on heparin
(3) mixing studies do not indicate an inhibitor (fully or partially corrected)
(4) Factor 2 levels are decreased
(5) depressed prothrombin functional activity
(6) if available, crossed immunoelectrophoresis shows an abnormal mobility
Differential diagnosis:
(1) lupus anticoagulant effect without reduced prothrombin: may increase the prothrombin time up to 3 seconds
(2) DIC (positive fibrin degradation products and hypofibrinogenemia)
(3) inhibitor to Factors 5, 7 or 10 (positive mixing studies)
(4) improper warfarin dosing (based on patient history; returns to normal on cessation of warfarin dosing or when shifted to heparin)
