Objective: Thrombosis is a relatively common complication in patients with systemic lupus erythematosus (SLE) and is strongly associated with the presence of antiphospholipid antibodies (aPL). The mechanism involved in the pathogenesis of this prothrombotic state remains obscure. We studied 4 natural anticoagulant proteins: protein C, protein S, antithrombin III, and plasminogen in 50 patients diagnosed with SLE.
Methods: Protein C, antithrombin III, and plasminogen were measured by chromogenic substrates and total and free protein S by electrophoresis. We also determined the prevalence of different aPL (lupus anticoagulant and antibodies against cardiolipin, phosphatidylserine, and phosphatidylinositol).
Results: Ten patients (20%) had a history of thrombosis. Some type of aPL was present in 26 patients (52%). Nine of the 10 patients with history of thrombosis had aPL (p = 0.007). Functional assays for protein C, antithrombin III, and plasminogen were in the normal range in all patients. Low free protein S levels were documented in 19 patients and were associated with the presence of aPL (13/19 were aPL positive) (p < 0.05). Only 4 patients with acquired free protein S deficiency had a history of thrombosis.
Conclusion: This study shows an association between aPL and reduced free protein S levels in patients with SLE. Further studies are needed to determine the mechanism and role of this acquired deficiency in the pathogenesis of thrombotic episodes in patients with SLE.