Prothrombin IgG antibodies measurement is a second-line test when seronegative antiphospholipid syndrome (APS) is strongly suspected.
Anti-prothrombin antibodies are autoantibodies produced by the immune system that target prothrombin, a protein involved in the blood clotting process. These antibodies are associated with certain autoimmune disorders, particularly antiphospholipid syndrome (APS), which is an autoimmune disorder that can cause abnormal blood clotting.
Antiphospholipid syndrome is characterized by the presence of antiphospholipid antibodies, which includes anti-cardiolipin antibodies, lupus anticoagulant, and anti-beta-2 glycoprotein I antibodies, in addition to anti-prothrombin antibodies. These antibodies can lead to an increased risk of blood clots in veins and arteries, as well as complications during pregnancy such as recurrent miscarriages.
The diagnosis of antiphospholipid syndrome requires that at least 1 clinical and 1 laboratory criterion be met. Clinical criteria include vascular thrombosis (arterial or venous in any organ or tissue) and pregnancy complications (unexplained fetal death, premature birth, severe preeclampsia, or placental insufficiency). Other clinical manifestations that are often associated with the syndrome, such as heart valve disease, peliosis hepatis, thrombocytopenia, nephropathy, and neurological symptoms, however, are not included in the diagnostic criteria. Laboratory criteria for the diagnosis of antiphospholipid syndrome are the presence of lupus anticoagulant, the presence of IgG and/or IgM cardiolipin antibodies, and the presence of IgG and/or IgM antibodies against beta-2-GP1. All antibodies should be positive for 2 or more times at least 12 weeks apart. Antibodies against beta-2-GP1 are somewhat more specific (but less sensitive) for the diagnosis of antiphospholipid syndrome. IgA antibodies against cardiolipin and beta-2-GP1 are not part of the laboratory criteria because of their low specificity.