C3 and systemic lupus erythematosus: At that time, she was diagnosed with APS secondary to SLE because of the repeated detection of high titers of antibody to double-stranded DNA (300 U/ml) and aCL IgG (39.6 U/ml) and low serum complement 3 (C3; 34 mg/dl) and complement 4 (C4; 2 mg/dl) concentrations.