Antithrombin III in systemic lupus erythematosus.

Plasma antithrombin III (AT III) was studied in 39 patients with systemic lupus erythematosus (SLE) and in 12 patients with other connective tissue disorders. AT III was measured immunologically by the Mancini method as well as by functional assay using thrombin and the chromogenic substrate, chromozyn TH (Boehringer). Reduced AT III activity was found in 17 patients; 8 had thrombosis. In 6 patients low AT III correlated with disease exacerbations and 2 had systemic vasculitis. No significant correlation could be demonstrated between low AT III levels and thromboembolic disease. A marked variation of functional AT III activity was observed in 30 patients in whom the presence of the lupus anticoagulant was demonstrated. The significance of this association is discussed.