Antiphospholipid syndrome (APS) is an uncommon but important cause of thrombosis and pregnancy morbidity. Despite guidelines, antiphospholipid antibody (aPL) testing is frequently performed outside specialist settings, potentially reducing diagnostic yield and leading to unnecessary downstream investigations. We evaluated real-world aPL testing practices, diagnostic yield, and clinical outcomes. We conducted a retrospective cohort study across three acute hospitals, two satellite sites and 63 general practices between February and June 2024. Adults undergoing aPL testing (anticardiolipin, β2-glycoprotein-I or lupus anticoagulant) were identified from electronic pathology records. Appropriateness, completeness, results and downstream clinical outcomes were assessed against BSH/EULAR guidance and the documented clinical context. Among 691 patients tested (mean age 51.3 years; 67% women), only 34% requests were considered appropriate, and complete panels were requested in 26%. APS was confirmed in 14 patients (2%). Appropriate requests were associated with higher rates of positive results [risk ratio (RR) 2.39], APS diagnosis (RR 27.0) and management changes (RR 21.4) (all P < 0.001). Haematology, Obstetrics and Rheumatology demonstrated the highest rates of appropriate testing. aPL testing is frequently performed outside recommended clinical indications, resulting in low overall diagnostic yield. Appropriate testing is associated with improved diagnostic accuracy and clinically meaningful changes in patient management, supporting the implementation of targeted diagnostic stewardship interventions.