Prothrombin IgA ELISA is a solid phase enzyme immunoassay employing native human prothrombin (Factor II) for the quantitative detection of antibodies against prothrombin in human serum. The assay is a tool in the diagnosis of the antiphospholipid syndrome. Human Factor II (prothrombin) is a plasme zymogen with a molecular weight of 72 kDa. It assembles with the activated forms of Factor V, Factor X and phospholipid to form a catalytic unit known as the prothrombinase complex. In the presence of calcium ions, the complex cleaves the membrane-associated prothrombin into thrombin, which is then released into the soluble phase. Antibodies against prothrombin belong to the group of antiphospholipid antibodies specific for negatively charged phospholipids, components of biological membranes, and prothrombin. The presence of anti-phospholipid antibodies has been associated with the clinical features of the so-called antiphospholipid syndrome (APS). Many studies have shown a correlation between these autoantibodies and an enhanced incidence of thrombosis, thrombocytopenia and habitual abortions (as a consequence of placental infarct). The exact mechanism by which pathogenic anti-phospholipid antibodies induce thrombosis is not yet revealed fully. The occurrence of anti-phospholipid antibodies in patients with SLE and related diseases is typical for a secondary anti-phospholipid syndrome (APS). In contrast, anti-phospholipid antibodies in patients with no other autoimmune diseases characterize the primary anti-phospholipid syndrome (APS).