The body's blood is normally in a liquid state. When a person is injured or has surgery, blood thickens and plugs up the spot that is bleeding in a process called hemostasis, also known as coagulation or clotting. Hemostasis is a normal, vital function of the body.

Sometimes in lupus, however, the processes of hemostasis are too strong, and a blood clot forms where it is not needed -- or wanted. This condition is called thrombosis. The difference between hemostasis and thrombosis is that the latter is too much of a good thing. If a thrombus, or clot, breaks off and travels elsewhere in the circulation, it is called an embolus.

Thromboembolism is not common, but it can be dangerous: Blood clots may affect the leg veins (sometimes with embolism going to the lungs), or the arteries to the arms, legs, or brain, as well as other places in the body.

During pregnancy, blood clots can lodge in the placenta and disrupt nutrition to the fetus. A baby may be born prematurely with low birth weight, or may not survive to be delivered. Some women lose pregnancies over and over until a proper diagnosis is made and treatment given.

Most thrombosis in lupus is associated with antibodies in the blood called antiphospholipid antibodies. The two blood tests most often used to detect antiphospholipids are the anticardiolipin test and the lupus anticoagulant test. (The lupus anticoagulant is not really an anticoagulant in the body–it just looks like one in the laboratory.)