An 18-year-old previously healthy man is placed on intravenous heparin after having a pulmonary embolism (PE) after exploratory laparotomy for a small-bowel injury following a motor vehicle collision. Five days later, his platelet count is 90,000/mL and continues to fall over the next several days. The patient's serum is positive for antibodies to the heparin-platelet factor complexes. Which of the following is the most appropriate next management step?

The patient has heparin-induced thrombocytopenia (HIT), which is a complication of heparin therapy, at both prophylactic and therapeutic doses. HIT is mediated by antibodies to the complexes formed by binding of heparin to platelet factor 4. In a previously unexposed patient, HIT typically manifests after 5 days as a decrease in platelet counts by 50% of the highest preceding value or to a level less than 100,000/mm3 . Complications of HIT are related to venous and/or aerial thromboembolic phenomena. Treatment of HIT consists of cessation of heparin (including low-molecular-weight heparins), institution of a nonheparin anticoagulant such as a direct thrombin inhibitor (examples include lepirudin and argatroban), and conversion to oral warfarin when appropriate. Cessation of heparin alone is inadequate to prevent thromboembolic complications, and warfarin should not be staed until the platelet count is above 100,000/mm3 . Platelet transfusion is not indicated, as HIT results in thrombotic rather than hemorrhagic complications.