Heparin is administered to > 12 million patients each year.3HIT is caused by antibodies that develop in up to 50% of patients following heparin exposure, against a complex of heparin and platelet factor 4.4Its occurrence is more frequent with bovine vs porcine heparin,5unfractionated vs low-molecular-weight heparin,6and after surgery.7These antibodies, circulating for ≥ 3 months in approximately 40% of patients,8 can activate platelets, leading to the release of procoagulant microparticles. In turn, this may cause excessive thrombin generation, thrombocytopenia, and clinical thromboses. HIT, thrombocytopenia due to the interaction of heparin, platelets, and platelet antibodies, is a profoundly prothrombotic condition occurring in 1 to 5% of patients receiving heparin,6 with up to 600,000 cases occurring yearly. With half of the patients in these cases developing HIT-related morbidity and mortality,9as many as 300,000 patients develop thrombotic complications, and 90,000 patients may die yearly. For perspective, there were 14,874 cases of tuberculosis reported to the Centers for Disease Control and Prevention in 2003,10an estimated 215,990 new cases of invasive breast cancer patients were reported in 2004 in the United States,11and cystic fibrosis affects 1 in 3,000 live births in the white population of North America (or about 3,200 babies)12 yearly.