• acquired activated protein C resistance;
  • anti-prothrombin antibodies;
  • lupus anticoagulant;
  • venous thromboembolism;
  • systemic lupus erythematosus

Summary. Venous thromboembolism (VTE) is one of the common manifestations in the anti-phospholipid (aPL) syndrome. We examined the levels of IgG antibodies (Abs) to β2-glycoprotein I (β2-GP I) and prothrombin, lupus anticoagulant (LA) activity, activated protein C resistance (APC-R), and factor V Leiden in 96 patients with systemic lupus erythematosus (SLE); 19 with VTE and 77 without VTE. Acquired APC-R, which was not found in any patient with the factor V Leiden mutation, was present in 33 (34·4%) out of the 96 patients with SLE. The presence of acquired APC-R was a strong risk factor for VTE. The SLE patients were divided into four groups according to the results of enzyme-linked immunosorbent assay (ELISA) and LA activity for each aPL Abs: ELISA+, LA+; ELISA+, LA; ELISA, LA+; and ELISA, LA. A significant association was observed between APC-R and the co-existence of anti-β2-GP I Abs and LA activity or of anti-prothrombin Abs and LA activity. There was no association between APC-R and the presence of anti-β2-GP I Abs, anti-prothrombin Abs, or LA activity alone. However, when multivariate logistical regression analysis was performed, it was clear that only the co-existence of anti-prothrombin and LA activity was a significant risk factor for APC-R. These findings indicate that the co-existence of anti-prothrombin Abs and LA activity may be an important factor in the pathogenesis of acquired APC-R in patients with SLE.