Abstract: Objectives: The 2023 ACR/EULAR criteria aim to improve the classification of antiphospholipid syndrome (APS). This study aims to validate these criteria and compare clinical and laboratory domains between classified and non-classified patients. Methods: A retrospective cohort study was conducted at the Hospital Universitario Marqués de Valdecilla, a tertiary referral center in Cantabria, Spain. Data were retrieved from the laboratory information system, identifying patients with at least one positive determination of lupus anticoagulant (LA), anticardiolipin (aCL), or anti-?2 glycoprotein I (anti-?2GPI) between January 2018 and March 2024. Patients were classified according to the 2006 Sydney and 2023 ACR/EULAR criteria. Results: Among 375 patients meeting the Sydney criteria, 152 (40.6 %) fulfilled the 2023 ACR/EULAR criteria. The sensitivity and specificity of the 2023 ACR/EULAR criteria were 30.2 and 97.7 % respectively, with a positive predictive value of 0.84, and a negative predictive value of 0.77. Area under the ROC curve was 0.639 (95 % CI: 0.605-0.673). The exclusion of thrombosis with high-risk thrombotic profiles and recurrent pregnancy loss or fetal death as individual events on obstetric APS accounted for most declassified patients. Additionally, patients meeting the 2023 ACR/EULAR criteria had a higher prevalence of arterial thrombosis without cardiovascular risk factors. In laboratory domains, isolated IgM aPL positivity was a major exclusion factor due to its lower weight in the new criteria. Conclusions: The 2023 ACR/EULAR criteria enhance specificity but significantly reduce sensitivity, excluding many APS patients, particularly those with obstetric APS or IgM aPL. This raises concerns about clinical trial eligibility and applicability in diverse populations.

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