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Objectives Anti-phosphatidylserine/prothrombin antibodies (aPS/PT) are non-criteria antiphospholipid antibodies (aPL) associated with thrombosis and pregnancy morbidity. The 2023 ACR/EULAR classification criteria highlight the need to evaluate non-criteria aPL, including aPS/PT, to guide future updates. We aimed to identify patients with isolated aPS/PT positivity and to characterize their clinical manifestations.Methods We prospectively enrolled 79 patients from 2011 to 2025 at the University Medical Centre Ljubljana and the University of Padua. IgG and IgM anticardiolipin (aCL), anti-βglycoprotein I (anti- β2GPI), and aPS/PT antibodies were measured at UMCLJ using in-house ELISAs (1) and at UniPD using commercial assays from Orgentec Diagnostika and INOVA (2). Lupus anticoagulant (LAC) was determined in a three-step procedure according to the ISTH guidelines.Results The cohort comprised 79 patients (mean age 45.5 ± 16.1 years, range 10-77 years, 16.5% men) persistently single aPS/PT positive (IgG and/or IgM), confirmed by at least two measurements. Importantly, 33/79 (42%) achieved > 3 clinical weight points (WP), fulfilling the clinical domain of the 2023 criteria ( figure 1). Among them, 14 were LAC-negative (not meeting the laboratory criteria), whereas 19 patients were LAC-positive and met the criteria (table 1). Mean clinical scores did not differ significantly between LAC-negative (4.1 WP) and LAC-positive patients (5.2 WP).Autoimmune disease, predominantly SLE, was present in 71% of LAC-negative and 68% of LAC-positive patients. In the LAC-negative group, 10 experienced venous thromboembolism (one high-risk), three arterial thrombosis (two with high-risk cardiovascular profile), one microvascular involvement, three obstetric manifestations (two preeclampsia), one cardiac-valve abnormality, and three thrombocytopenia. Among LAC-positive patients, 11 had venous thrombosis (two high-risk), eight arterial thrombosis (one high-risk), six microvascular manifestations (four suspected, two confirmed), four obstetric complications, four valvular lesions, and five thrombocytopenia.Abstract LBA:01:03 Figure 1Conclusions Single aPS/PT positivity is associated with all six clinical domains included in the 2023 ACR/EULAR APS classification criteria, even independently of LAC positivity. The association of aPS/PT with newly added clinical domains, known to be linked to damage accrual, makes aPS/PT a tool for risk stratification in APS.