First‐Line, Non‐Criterial Antiphospholipid Antibody Testing for the Diagnosis of Antiphospholipid Syndrome in Clinical Practice: A Combination of Anti–β 2 ‐Glycoprotein I Domain I and Anti–Phosphatidylserine/Prothrombin Complex Antibodies Tests

Objective To assess the value of a combination of anti–β 2 ‐glycoprotein I (anti‐β 2 GPI ) 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‐β 2 GPI 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‐β 2 GPI 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‐β 2 GPI 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‐β 2 GPI 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–β 2 GPI 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.