Laboratory Testing for Antiphospholipid Syndrome

The antiphospholipid syndrome (APS) is an autoimmune condition characterized by arterial and venous thrombosis, recurrent fetal loss, thrombocytopenia, and livedo reticularis. This chapter describes the nature of the laboratory tests in detail, their historical development, methodology including standardization, specificity, sensitivity, association with clinical phenotypes, and clinical utility. Persistent positivity for aPL in conjunction with clinical events forms the cornerstone for the diagnosis of APS. The three laboratory tests that are in common and universal use are the aCL, LA, and antiβ2GPI antibody tests. However, standardization and interlaboratory variability continue to be issues, despite universal calibrators being employed for aCL testing, for example. Despite this, persistent positivity for these tests, particularly when triple positive, has a high association with APS disease. Of the three tests, persistent LA positivity probably harbors the greatest association with pathogenicity, particularly when antiβ2GPI positive. However, given the high percentage of aPL positivity in the normal population, the risk of developing a clinical event in the setting of persistent aPL positivity remains largely unknown due to the lack of prospective studies designed to address this question.