Value of autoantibodies to β2‐glycoprotein 1 in the diagnosis of antiphospholipid syndrome

Objectives. To define the specificity and positive predictive value of anti‐β2‐glycoprotein 1 (anti‐β2GP1) antibodies for the diagnosis of antiphospholipid syndrome (APS). Methods. We determined the presence of anticardiolipin (aCL) antibodies and anti‐β2‐glycoprotein 1 (anti‐β2GP1) immunoglobulin (Ig) G and IgM in 191 consecutive sera from 191 patients and reviewed clinical data separately. aCL IgG and IgM were detected separately using commercial ELISA kits. Anti‐β2GP1 antibodies were detected with an in‐house ELISA using β2GP1. Results. Seven patients were diagnosed as having APS and 184 as having other diseases. Thirty‐six patients were aCL‐positive and 12 were anti‐β2GP1‐positive, seven of these 12 were APS patients. The specificity for anti‐β2GP1 in our population was 97%, with a positive predictive value (PPV) of 58%. Among the aCL‐positive patients, specificity was 90% and PPV 70–87%. Conclusions. This study shows that anti‐β2GP1 antibodies have a higher specificity and PPV than aCL for APS. The PPV of anti‐β2GP1 was greater in aCL‐positive than in all patients. We conclude that screening for anti‐β2GP1 antibodies in aCL‐positive patients increases the specificity and the PPV of aCL testing. In addition, we show that there is no need to screen for anti‐β2GP1 antibodies in the absence of aCL antibodies and in the absence of strong clinical suspicion of APS.