<h3>Background</h3> High levels of antiphospholipid antibodies (aPL) are associated predominantly with a higher risk of thrombosis, however, information about differential levels according underlying diagnosis is less well known. <h3>Objectives</h3> Our aim was to compared serum levels of 2 novel aPL (anti-phosphatidylserine/prothrombin (PS/PT) antibodies and anti domain 1 against ß2 glycoprotein I (anti-D1 B2GP1)) and "classic" (anticardiolipin, aCL and anti B2GP1 antibodies) among patients with primary APS, SLE with and without thrombosis <h3>Methods</h3> In this cross-sectional study, Anti-D1 B2GPI antibodies were tested using a chemiluminescent immunoassay (QUANTA Flash, Inova Diagnostics). In addition, anti PS/PT (IgG and IgM), aCL (IgG and IgM) and anti B2GP1 (IgG and IgM) were measured at the same time by ELISA techniques (QUANTA Lite, Inova Diagnostics). Lupus anticoagulant was detected according to the guidelines of the International Society on Thrombosis and Haemostasia. Differences in mean values between groups were analysed with the Mann-Whitney U test for comparison of 2 groups <h3>Results</h3> One hundred and seventy three patients were included, 87% of patients were female with a mean age of 33±12 years. 150 patients had SLE and 23 patients primary APS. Fifty five (31%) out of 177 had history of thrombosis and 41 (23%) of pregnancy losses. Levels of IgM aCL, IgM B2GP1, IgM Anti PS/PT and total antibodies against Anti-D1 B2GP1 were significantly higher in patients with primary APS than in patients with SLE and thrombosis (<b>see figure 1</b>).IgG anti PS/PT (75.9±91.3 vs 27.4±39.3 IU, p&lt;0.001) and total anti-D1 B2GP1 (65.2±152 vs 16.4±65.1 IU, p&lt;0.001) were significantly higher in patients with SLE with thrombosis than in patients with SLE without thrombosis. Obstetric morbidity (66.7% vs 21.7%, p=0.016) and <i>livedo reticularis</i> (22% vs 0%, p&gt;0.07) were more common in patients with primary APS than SLE with thrombosis. No significant differences were found in terms of thrombotic events among both groups <h3>Conclusions</h3> Patients with primary APS have significant higher serum levels of anti PS/PT and anti-D1 B2GP1 antibodies and "classic" aPL (aCl and anti B2GP1) antibodies than patients with SLE with thrombosis. Whether if these titers are useful to differentiate patients with primary and secondary APS requires further analysis in prospective studies. <h3>Acknowledgements</h3> Anti D1- B2GPI antibodies antibodies were provided by Inova, Werfen, Colombia <h3>Disclosure of Interest</h3> J. A. Gómez-Puerta Speakers bureau: Werfen/Inova, T. Urrego: None declared, A. Hernández: None declared, S. Ruiz: None declared, S. M. Osorno: None declared, C. Rua: None declared, J. Duque Botero: None declared, A. Vanegas-García: None declared, C. H. Muñoz: None declared, L. A. González: None declared, G. Vásquez: None declared