In vitro correction of anticoagulant activity and specific clotting factor assays in SLE

Six patients with circulating anticoagulants, four of whom had proven SLE, were investigated. The plasma of five patients demonstrated ostensibly low specific clotting factor activities by standard PTT methods which were corrected by the addition of from four to eight times the usual concentration of phospholipid. Normal factor VIII levels were obtained in each case when a two-stage clotting assay was employed and normal factor IX activities were measured when heterologous substrate was substituted for deficient plasma of human origin. Specific immunoglobulin fractions of whole serum localized the clotting inhibitory activity of the PTT system to the IgG fraction of one patient and the IgG and IgM fractions of another. The plasma of three patients inhibited platelet factor 3 activity and in three cases inhibition of rabbit but not human thromboplastin was observed. Inhibitors directed against the phospholipid components of the coagulation system may cause the appearance of specific factor deficiencies when PTT assays are employed. Such deficiences should not be reported as "true" factor deficiencies, until multiple assays have been performed that employ a related technique.