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

<jats:sec><jats:title>Objective</jats:title><jats:p>To assess the value of a combination of anti–β<jats:sub>2</jats:sub>‐glycoprotein I (anti‐β<jats:sub>2</jats:sub><jats:styled-content style="fixed-case">GPI</jats:styled-content>) domain I antibody and anti–phosphatidylserine/prothrombin complex (anti‐<jats:styled-content style="fixed-case">PS</jats:styled-content>/<jats:styled-content style="fixed-case">PT</jats:styled-content>) antibody tests for the diagnosis of antiphospholipid syndrome (<jats:styled-content style="fixed-case">APS</jats:styled-content>).</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>This cross‐sectional study involved a cohort of the patients who visited our clinic from April 2005 to March 2013. Tests for anti‐β<jats:sub>2</jats:sub><jats:styled-content style="fixed-case">GPI</jats:styled-content> domain I antibodies, IgG anti‐<jats:styled-content style="fixed-case">PS</jats:styled-content>/<jats:styled-content style="fixed-case">PT</jats:styled-content> antibodies, and IgM anti‐<jats:styled-content style="fixed-case">PS</jats:styled-content>/<jats:styled-content style="fixed-case">PT</jats:styled-content> antibodies, together with tests for criteria‐defined antiphospholipid antibodies (<jats:styled-content style="fixed-case">aPL</jats:styled-content>), were performed in all patients. The total antiphospholipid score (<jats:styled-content style="fixed-case">aPL</jats:styled-content>‐S) was calculated for each patient according to titers of and positivity for <jats:styled-content style="fixed-case">aPL</jats:styled-content>.</jats:p></jats:sec><jats:sec><jats:title>Results</jats:title><jats:p>The study enrolled 157 patients (51 patients with <jats:styled-content style="fixed-case">APS</jats:styled-content> and 106 with non‐<jats:styled-content style="fixed-case">APS</jats:styled-content> autoimmune diseases). All 21 patients positive for both anti‐β<jats:sub>2</jats:sub><jats:styled-content style="fixed-case">GPI</jats:styled-content> domain I antibodies and IgG and/or IgM (IgG/IgM) anti‐<jats:styled-content style="fixed-case">PS</jats:styled-content>/<jats:styled-content style="fixed-case">PT</jats:styled-content> antibodies had <jats:styled-content style="fixed-case">APS</jats:styled-content> with a high total <jats:styled-content style="fixed-case">aPL</jats:styled-content>‐S (median 46, range 26–76), as did all of the 10 patients who were positive for anti‐β<jats:sub>2</jats:sub><jats:styled-content style="fixed-case">GPI</jats:styled-content> domain I antibodies but negative for IgG/IgM anti‐<jats:styled-content style="fixed-case">PS</jats:styled-content>/<jats:styled-content style="fixed-case">PT</jats:styled-content> antibodies (median 22, range 4–39). Of the 14 patients who were positive for IgG/IgM anti‐<jats:styled-content style="fixed-case">PS</jats:styled-content>/<jats:styled-content style="fixed-case">PT</jats:styled-content> antibodies but negative for anti‐β<jats:sub>2</jats:sub><jats:styled-content style="fixed-case">GPI</jats:styled-content> domain I antibodies, 11 (79%) had <jats:styled-content style="fixed-case">APS</jats:styled-content>; these individuals also had high total <jats:styled-content style="fixed-case">aPL</jats:styled-content>‐S values (median 23, range 11–60). In contrast, only 9 of the 112 patients (8%) with none of these antibodies had APS.</jats:p></jats:sec><jats:sec><jats:title>Conclusion</jats:title><jats:p>The combination of the IgG anti–β<jats:sub>2</jats:sub><jats:styled-content style="fixed-case">GPI</jats:styled-content> domain I antibody and IgG/IgM anti‐<jats:styled-content style="fixed-case">PS</jats:styled-content>/<jats:styled-content style="fixed-case">PT</jats:styled-content> antibody tests shows a high positive predictive value for the diagnosis of <jats:styled-content style="fixed-case">APS</jats:styled-content> and a strong correlation with the <jats:styled-content style="fixed-case">aPL</jats:styled-content>‐S. This combination as the first‐line test for <jats:styled-content style="fixed-case">aPL</jats:styled-content> may contribute to the simple and definite identification of <jats:styled-content style="fixed-case">APS</jats:styled-content> with a high risk of thrombosis in clinical practice.</jats:p></jats:sec>