Association of autoantibodies against the phosphatidylserine–prothrombin complex with manifestations of the antiphospholipid syndrome and with the presence of lupus anticoagulant

Objective To clarify the association of autoantibodies against prothrombin with the clinical manifestations of the antiphospholipid syndrome (APS) and with the presence of lupus anticoagulant (LAC). Methods We examined 265 patients who visited our autoimmune disease clinic. IgG and IgM antiprothrombin antibodies were tested by enzyme‐linked immunosorbent assay (ELISA) as either antiphosphatidylserine–prothrombin complex (aPS/PT) antibodies or as antibodies against prothrombin coated on irradiated ELISA plates (as antigen) (aPT). IgG, IgM, and IgA anticardiolipin (aCL) antibodies and their β 2 ‐glycoprotein I (β 2 GPI) dependency were also evaluated by ELISA. LAC was tested by 3 different methods. Results The presence of aPS/PT, but not of aPT, significantly correlated with the clinical manifestations of APS (odds ratio [OR] 4.39, 95% confidence interval [95% CI] 2.06–9.38), and aPS/PT antibodies were as specific as β 2 GPI‐dependent aCL for APS (93.1% for both). IgG aPS/PT strongly correlated with the presence of LAC as detected using the dilute Russell viper venom time test (OR 38.2, 95% CI 13.4–109.1). Conclusion Antiprothrombin antibodies are heterogeneous and their clinical relevance depends on the method of detection applied. Positive results on the aPS/PT test can serve as a marker of thrombotic events in patients with autoimmune diseases.