Antiphospholipid syndrome: epidemiology and outcomes in the evolving criteria era

Antiphospholipid syndrome (APS) is a heterogeneous autoimmune thrombo-inflammatory disorder characterized by thrombosis, pregnancy morbidity and an expanding spectrum of microvascular, cardiac, haematological and other systemic manifestations. Although research has provided considerable insights into the pathogenesis and clinical phenotypes of APS, its epidemiology remains difficult to define because estimates depend on case definitions, antibody testing, repeat-testing requirements, testing practices and data sources. Population-based studies suggest that APS is rare, and catastrophic APS is rarer and best understood through registry data. Antiphospholipid antibody positivity alone does not establish APS; transient or low-level positivity can occur without disease, whereas persistent antibodies must be interpreted alongside compatible clinical manifestations. Despite its rarity, APS causes substantial morbidity through recurrent thrombosis, organ damage and adverse pregnancy outcomes, supporting targeted evaluation in unprovoked or recurrent venous thromboembolism, stroke in younger adults and selected pregnancy complications. Apparent differences among populations can reflect ascertainment, access to testing, study design and biological variation. Standardized case definitions, antibody testing and laboratory reporting, together with longitudinal population-based research in ancestry-diverse, paediatric and low-income and middle-income populations, are needed to refine estimates of APS burden and define its clinical spectrum.