First-Line, Non-Criterial Antiphospholipid Antibody Testing for the Diagnosis of Antiphospholipid Syndrome in Clinical Practice: A Combination of Anti–β₂-Glycoprotein I Domain I and Anti–Phosphatidylserine/Prothrombin Complex Antibodies Tests

Objective: To assess the value of a combination of anti–β₂-glycoprotein I (anti-β₂GPI) domain I antibody and anti–phosphatidylserine/prothrombin complex (anti-PS/PT) antibody tests for the diagnosis of antiphospholipid syndrome (APS). Methods: This cross-sectional study involved a cohort of the patients who visited our clinic from April 2005 to March 2013. Tests for anti-β₂GPI domain I antibodies, IgG anti-PS/PT antibodies, and IgM anti-PS/PT antibodies, together with tests for criteria-defined antiphospholipid antibodies (aPL), were performed in all patients. The total antiphospholipid score (aPL-S) was calculated for each patient according to titers of and positivity for aPL. Results: The study enrolled 157 patients (51 patients with APS and 106 with non-APS autoimmune diseases). All 21 patients positive for both anti-β₂GPI domain I antibodies and IgG and/or IgM (IgG/IgM) anti-PS/PT antibodies had APS with a high total aPL-S (median 46, range 26–76), as did all of the 10 patients who were positive for anti-β₂GPI domain I antibodies but negative for IgG/IgM anti-PS/PT antibodies (median 22, range 4–39). Of the 14 patients who were positive for IgG/IgM anti-PS/PT antibodies but negative for anti-β₂GPI domain I antibodies, 11 (79%) had APS; these individuals also had high total aPL-S values (median 23, range 11–60). In contrast, only 9 of the 112 patients (8%) with none of these antibodies had APS. Conclusion: The combination of the IgG anti–β₂GPI domain I antibody and IgG/IgM anti-PS/PT antibody tests shows a high positive predictive value for the diagnosis of APS and a strong correlation with the aPL-S. This combination as the first-line test for aPL may contribute to the simple and definite identification of APS with a high risk of thrombosis in clinical practice.