CO114 | ANTI-PS/PT IgM IDENTIFIES AN ANTIGEN-DEFINED SUBSET WITHIN LUPUS ANTICOAGULANT POSITIVITY ASSOCIATED WITH THROMBOTIC PHENOTYPE

M. Sacco¹, S. Lancellotti¹, A. Ferretti¹, M. Basso¹, C. Gavotti², A. Ortolan², V.A. Pacucci², P. Calzoni³, L. Puccetti³, R. De Cristofaro^1|4 | ¹Center for Hemorrhagic and Thrombotic Diseases; ²Rheumatology and Clinical Immunology Unit, Fondazione Policl. Univ. Gemelli IRCCS, Roma, Italy; ³Azienda Ospedaliera-Universitaria Senese, Unità specialistica Coagulopatie Trombotiche ed Emorragiche, Dipartimento di Scienze Mediche, Chirurgiche e Neuroscienze, Università di Siena, Siena, Italy; ⁴Department of Translational Medicine and Surgery, Catholic University of the Sacred Heart, Roma, Italy

Background and Aims. Interpretation of isolated lupus anticoagulant (LAC) positivity remains challenging because LAC represents a functional coagulation abnormality rather than an antigen-defined immune response. Anti-phosphatidylserine/prothrombin (aPS/PT) antibodies have been proposed as non-criteria antiphospholipid antibodies that may refine serological characterization of LAC-positive patients. The study aims to determine whether aPS/PT antibodies identify antigen-defined subgroups associated with thrombotic phenotype among consecutive patients undergoing antiphospholipid antibody testing.
Methods. We performed a retrospective two-center observational study including 131 consecutive adult patients referred for antiphospholipid antibody evaluation (Fig. 1). All underwent testing for LAC, anticardiolipin antibodies, anti-β2-glycoprotein I antibodies, and aPS/PT antibodies. Because antibody testing was performed after clinical events, thrombosis was analyzed as a historical clinical phenotype. Associations were assessed using stratified comparisons and logistic regression adjusted for age, sex, smoking status, and hypertension.
Results. A history of objectively confirmed thrombosis was present in 72.1% of patients. aPS/PT IgM levels correlated with LAC activity measured by the dRVVT ratio. Patients positive for LAC and aPS/PT IgM but negative for anticardiolipin and anti-β2-glycoprotein I antibodies had significantly higher odds of prior thrombosis compared with the reference serological profile (adjusted OR 11.2; 95% CI 2.5–50.8; p = 0.002).
Conclusions. In this selected tertiary referral cohort enriched for thrombotic disease, aPS/PT IgM identified a biologically and clinically distinct subset within LAC-positive patients associated with prior thrombosis. These findings support further prospective evaluation of aPS/PT testing for refinement of heterogeneous LAC positivity.