MEDizzy - A 58-year-old man with a history of deep venous thrombosis

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Hemodynamics

A 58-year-old man with a history of deep venous thrombosis following right total knee replacement is recovering from multiple injuries sustained in a motor vehicle collision. His injuries include fractures to both tibial plateaus. He is on subcutaneous unfractionated heparin for prevention of deep venous thrombosis. On his 6th hospital day his platelet count dropped from 250,000/µL to 110,000/µL. He is otherwise recovering well with no complications, and a lower extremity Doppler is performed, which is negative for deep venous thrombosis. Which of the following additional tests would be most helpful in determining the cause of his thrombocytopenia?

ExplanationThe > 50% drop in the platelet count is suggestive of heparin-induced thrombocytopenia, and his prior treatment for deep venous thrombosis would likely have meant prior exposure to heparin. Heparin-induced thrombocytopenia is caused by antibodies to heparin-platelet factor-4 complexes.