Laboratory Tests for the Antiphospholipid Syndrome

Antiphospholipid syndrome (APS) is an autoimmune disorder characterized by recurrent vascular thrombosis (VT) and/or pregnancy morbidity (PM) in the presence of persistent antiphospholipid antibodies (aPL), detected by lupus anticoagulant (LA), anticardiolipin (aCL) antibody, and/or anti‐β₂ glycoprotein I (aβ₂ GPI) antibody assays. These aPL, considered to be diagnostic markers and pathogenic drivers of APS, are a heterogeneous group of antibodies directed against anionic phospholipids, phospholipid-binding plasma proteins, and phospholipid–protein complexes. Although APS is currently considered as a single disease, it presents with a wide range of clinical symptoms and biological characteristics. The clinical diagnosis of APS in a patient with symptoms and signs is dependent upon the presence of a persistently positive result in an aPL assay. The tests recommended for detecting aPL are the standardized enzyme-linked immunosorbent assay (ELISA) to detect aCL and aβ₂ GPI and clotting assays for LA performed according to the guidelines of the International Society on Thrombosis and Haemostasis [1, 2]. This chapter describes the standard laboratory test for the diagnosis of APS discussing the clinical and theoretical aspects of LA, aCL, and aβ₂ GPI assays.