The lupus inhibitor: a study of its heterogeneity.

The plasmas of six patients with prolonged activated partial thromboplastin times were studied in detail. In five of the six, the Russell's viper venom and prothrombin times were likewise prolonged. Five of the patients had documented systemic lupus erythematosus; one lacked the necessary criteria for this diagnosis. On quantitation, factor XI was decreased in all six; factors X and XII were diminished in five of the six. When tested for inhibitory activity, plasma from each of the patients prolonged the celite eluate inhibition test for factor XII and/or XI inhibition. In the formation of the Xa-V-phospholipid-Ca2+ complex (prothrombinase), factors X and Xa were inhibited to a greater degree than factor V or the phospholipid. Finally, each plasma was isofocused, the inhibitory fractions were identified and the clotting factor specificity of each inhibitory peak was determined. Fractions inhibitory against factors XI and XII isofocused with the IgG in each patient's plasma. Based on the data presented from these six patients, the "lupus inhibitor" is in fact a heterogeneous collection of inhibitors directed against factors XII, XI and X rather than a homogeneous entity.