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Objectives To evaluate the performance of the 2023 ACR/EULAR classification criteria for antiphospholipid syndrome (APS) in a real-world cohort, with particular focus on patients with a clinical diagnosis of APS who did not meet the classification criteria.Methods We analyzed 100 patients followed at Karolinska University Hospital, including 87 patients diagnosed with APS by a panel of experienced physicians and 13 asymptomatic antiphospholipid antibodies (aPL) carriers. The sensitivity and specificity of the 2023 ACR/EULAR criteria were calculated against the physicians’ consensus diagnosis based on comprehensive clinical assessment, laboratory findings, and longitudinal follow-up, as the reference standard. Patients not fulfilling classification were further analyzed to identify reasons for exclusion.Results The 2023 classification criteria demonstrated a sensitivity of 68.9% and a specificity of 84.6% in differentiating APS patients from aPL carriers in our cohort(Fig.1). Among the clinically diagnosed APS cases, 27 did not meet classification: 7 failed to reach any clinical or laboratory thresholds; 8 were excluded based on the clinical domain(4 had obstetric manifestations,2 arterial thrombosis,1 microvascular thrombosis, and 1 thrombocytopenia); 12 were excluded by laboratory domain(1 with isolated IgM positivity, 1 with lupus anticoagulant on a single occasion, 2 had low aPL titers, 8 were negative for all ELISA tests).The lowest sensitivity was observed in the obstetric APS (OAPS) subgroup: of 25 with physician-diagnosed OAPS, only 14(56%) met classification. The remaining 11 were excluded due to low scoring weighting of fetal losses(n=3), low-titer antibody profiles despite the strong clinical association with pregnancy morbidity(n=3), or insufficient points for both clinical and laboratory domains(n=5). Notably, all five patients in the latter group tested positive for aPL by multiplex assays, with four demonstrating high titers.Abstract PO:01:02 Figure 1Sensitivity and Specificity of the 2023 ACR/EULAR Classification Criteria in APS Subgroups and aPL carriersConclusions The 2023 ACR/EULAR criteria demonstrate good specificity but only moderate sensitivity in real-world APS cohorts, with the greatest limitations in OAPS. The underrepresentation of obstetric manifestations and exclusion of isolated IgM may limit the inclusion in research studies of patients with clinically relevant APS. Moreover, differences between multiplex and ELISA results highlight how assay-dependent variability can affect antibody detection and classification. Addressing these gaps, could improve the criteria’s ability to capture clinically relevant APS cases while preserving high specificity.