Heparin-induced thrombocytopenia (HIT), a condition characterized by low platelet levels and possible blood clots, occurs in a small number of people after treatment with the drug heparin. Some people with HIT may show symptoms of a blood clot at the time of HIT diagnosis, but in another form of HIT, known as isolated HIT, people do not show blood clot symptoms even though they might have a blood clot. This study will use ultrasound tests to evaluate the presence of blood clots at the time of an HIT diagnosis and in the following month.