Phospholipids are fats that contain phosphorus. Antiphospholipid (APL) antibodies are acquired antibodies to phospholipid-bound protein and prothrombin. These antibodies can prolong coagulation times. APL antibodies occur in about 5% of the population, including healthy persons and are common in people with lupus erythematosus (10–30%) other connective tissue disorders, cancer, HIV, drug and possibly estrogen intake, pulmonary embolism, intravenous catheter thrombosis, or thrombosis in the liver, portal vein, kidney, or retina.

A person with APL antibodies who has symptoms needs to take heparin for the acute event, then long-term warfarin. Some physicians prescribe steroids also.

A person with APL antibodies who has lupus (lupus anticoagulant) and no symptoms and is having vascular reconstruction should take antiplatelet therapy such as aspirin preoperatively, heparin therapy during the operation, and warfarin postoperatively.

Activated protein C cleaves (divides) and inactivates coagulation factors Va and VIIIa in the presence of protein S. This system allows blood to clot while maintaining fluidity. A genetic change in factor V, called factor V Leiden, causes factor V to be resistant to protein C and can result in clotting.

Diagnosis is by a blood test that measures the responsiveness of plasma to coagulation factor V Leiden or by direct assay for factor V molecule that is resistant to inactivation by factor V Leiden.

Treatment of patients who have had thromboses is long-term warfarin.