Phosphatidylserine-dependent antiprothrombin antibodies (aPS/PT) are strongly correlated with lupus anticoagulant. We evaluated the value of IgG aPS/PT for diagnosing antiphospholipid syndrome (APS). Methods: We performed an initial cross-sectional multi-centre study involving 8 centres/7 countries. Clinical and laboratory data were retrospectively collected. Specimens were blinded,and IgG aPS/PT determinations performed at Inova Diagnostics (USA, Inova) using 2 ELISA kits: MBL (Japan) and Inova. A validation study was carried out (5 centres/5 countries). Results: In the initial study (n = 247), IgG aPS/PT titers were concordant between the two ELISA tests (r = 0.827, p < 0.001). In samples with concordant results (n = 204), IgG aPS/PT were more prevalent in APS patients (51%) than in patients without APS (9%) (OR: 10.8 [95%CI 4-29], p < 0.0001). In the validation study (n = 214), there was as well a good concordance between IgG aPS/PT titers obtained by both ELISAs (r = 0.803, p < 0.001) , and IgG aPS/PT were more frequently found in APS patients. Conclusions: Performance of IgG aPS/PT is reliable. IgG aPS/PT detection is an easily performed laboratory parameter that may help in APS diagnosis.