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Summary.  Antiphospholipid syndrome (APS) is a clinical autoimmune disorder characterized by thrombosis/pregnancy morbidity associated with the persistence of lupus anticoagulant (LA) and/or anticardiolipin (aCL) antibodies. We assessed the contribution of antibodies to β2‐glycoprotein I (anti‐β2GPI) and prothrombin (anti‐PT) to the thrombotic risk in a cohort of 194 consecutive patients with persistent LA and/or aCL. Median follow‐up was 45 months. A total of 39 patients (20.1%) had one documented episode of thrombosis during follow‐up. Eleven of these patients had no previous thrombosis before enrollment in the study and 28 had recurrences of thrombosis. There were 21 venous and 18 arterial thrombotic events and the overall incidence of thrombosis was 5.6% per patient‐year. After multivariate analysis, the male sex (P = 0.025), a previous thrombosis (P < 0.01), the presence of anti‐β2GPI (P = 0.001), and the presence of anti‐PT (P = 0.03) remained as independent risk factors for recurrent thrombosis. Only IgG anti‐β2GPI and anti‐PT were associated with an increased risk of thrombosis (P < 0.01 and P = 0.005). Patients testing positive for anti‐β2GPI had a higher rate of thrombosis than did antiphospholipid patients without anti‐β2GPI (8.0% vs. 3.1% per patient‐year). Similarly, a higher rate of thrombosis was found in patients with positive anti‐PT compared with patients without anti‐PT (8.6% vs. 3.5% per patient‐year). Considering only the group of 142 LA positive patients, the highest incidence of thrombosis was found in LA patients positive for both anti‐β2GPI and anti‐PT (8.4% per patient‐year). In conclusion, the presence of IgG anti‐β2GPI and anti‐PT in patients with LA and/or aCL and mainly in those with LA predicts a higher risk of thromboembolic events.

Current classification criteria for definite APS recommend the use of one or more of three positive standardized laboratory assays, including anticardiolipin antibodies (aCL), lupus anticoagulant (LA), and antibodies directed to β(2)glycoprotein I (anti-β(2)GPI) to detect antiphospholipid antibodies (aPL) in the presence of at least one of the two major clinical manifestations (i.e., thrombosis or pregnancy morbidity) of the syndrome. Several other autoantibodies shown to be directed to phospholipids and/or their complexes with phospholipids and/or to proteins of the coagulation cascade, as well as a mechanistic test for resistance to annexin A5 anticoagulant activity, have been proposed to be relevant to APS. A task force of worldwide scientists in the field discussed and analyzed critical questions related to 'non-criteria' aPL tests in an evidence-based manner during the 13th International Congress on Antiphospholipid Antibodies (APLA 2010, 13-16 April 2010, Galveston, Texas, USA). This report summarizes the findings, conclusions, and recommendations of this task force.