Circulating Anticoagulants Against Factors IX and XI in Systemic Lupus Erythematosus

In four patients with systemic lupus erythematosus and unusual circulating anticoagulants the partial thromboplastin time of normal plasma was prolonged by the patient's plasma, but the prothrombin time was normal. Low plasma levels of factor IX in one case and of factor XI in two cases were the only coagulation abnormalities, suggesting that the anticoagulant was directed against these factors. In the fourth patient, assay values of factors V, VIII, IX, and X were not low enough to explain the prolonged partial thromboplastin time. Adequate contact product was found, but the patient's plasma had anticoagulant activity against her own and normal contact product. This interfered with assays of factors XI and XII; we believe that in this patient the anticoagulant was directed against activated factor XI. In two cases there was significant bleeding. Treatment with prednisone and immunosuppressive agents resulted in disappearance of the clotting abnormalities.