Antiphospholipid antibody tests are used to detect several specific phospholipid-binding proteins that the body produces against itself in an autoimmune response to phospholipids. Their presence increases the risk of developing recurrent inappropriate blood clots (thrombi) in arteries and veins, which can lead to strokes and heart attacks. Antiphospholipid antibodies are also associated with thrombocytopenia and with the risk of recurrent miscarriages (especially in the 2nd and 3rd trimester), premature labor, and pre-eclampsia. The most common antiphospholipid antibodies are cardiolipin antibodies and the lupus anticoagulant. Others that are not as frequently tested include anti-beta2 glycoprotein I and anti-phosphatidylserine. Antiphospholipid syndrome (APS) can be either primary, or secondary. It is “primary” when APS occurs in the absence of any other related disease, and “secondary” when seen in conjunction with other autoimmune diseases; such as systemic lupus erythematosus (SLE). In rare cases, APS leads to rapid organ failure due to generalised thrombosis and a high risk of death; this is termed "catastrophic antiphospholipid syndrome" (CAPS). Therefore, this test is used to help determine the cause of an unexplained thrombotic episode, recurrent fetal loss, thrombocytopenia, and/or a prolonged PTT test. Antiphospholipid antibodies may also be associated with HIV, some cancers, temporarily with infections, and with some drug treatments. Other related tests: Cardiolipin Antibodies, Serum; B-2 Glycoprotein I Antibodies, Serum; and Lupus Anticoagulant, Plasma.