Snapshot A 60-year-old woman presents after a 12 hour train ride with right-sided crampy leg pain. She is currently on hormone-replacement therapy for menopausal symptoms. On exam, the circumference on the right calf is larger than the left’s. An ultrasound shows a lower extremity DVT. She is started on heparin. Two days later, her platelets are measured at 30,000/mm3 which were previously at 150,000/mm3. On re-evaluation, her physician notes that she has several patches of purple/brown areas of skin necrosis. Concerned, he immediately stops the heparin and starts her on a direct thrombin inhibitor. Introduction Decreased platelets due to heparin exposure Type I HIT Two days after heparin exposure not immune-mediated platelet count normalizes spontaneously no symptoms Type II HIT hypercoagulable state resulting from antibody-mediated destruction of platelets 5-10 days after heparin associated with significant risk of thrombosis can be fatal epidemiology female > male (1.7x) pathogenesis heparin binds to platelet factor 4 (PF4) IgG antibodies recognize the heparin-PF4 complex type II hypersensitivity reaction complex-bound antibodies bind to platelets and cause platelet activation thrombosis thrombocytopenia risk factors more common with unfractionated heparin Presentation Symptoms (type II HIT) 5-10 days after heparin > 30% drop in platelets venous > arterial thrombosis DVT/PE skin necrosis overt bleeding is rare can see bleeding at injection sites Evaluation Complete blood count drop in platelet count by > 30% no matter what the absolute platelet count is ↑ bleeding time Normal PT/PTT Presence of anti-PF4-heparin for type II HIT serotonin release assay enzyme immunoassay Differential Diagnosis DIC ITP TTP HUS Treatment Type II HIT stop all heparin-containing products (including unfractionated and low molecular weight) direct thrombin inhibitors argatroban, lepirudin, and bivalirudin transition to warfarin when platelets are back at baseline PLT infusion may be used if bleeding and platelets <50,000/mm^3 (<100,000/mm^3 for intracranial bleed) not indicated if patient is not bleeding may worsen thrombosis Prognosis, Prevention, and Complications Prognosis mortality 5-10% in patients with type II HIT from thrombosis Complications thrombosis amputation of limb stroke