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. An ultrasound shows a lower extremity DVT. She is started on heparin. Two days later, her platelets come back at 24,000/mm3. They were previously 30,000/mm3. Her labs are monitored routinely, and her platelets returned to baseline after a few days. Introduction Decreased platelets due to exposure to heparin Epidemiology Female > male (1.7x) Risk factors more common with unfractionated heparin longer duration of heparin use etiology Type I HIT presents within first 2 days after exposure to heparin platelet count normalizes spontaneously non-immune mediated direct effect of heparin on platelet activation Type II HIT hypercoagulable state associated with significant risk of thrombosis can be fatal presents 5-10 days after exposure to heparin pathogenesis heparin binds to platelet factor 4 (PF4) IgG antibodies recognize the heparin-PF4 complex complex-bound antibodies bind to platelets and cause platelet activation thrombosis thrombocytopenia type II hypersensitivity reaction Presentation Type I - asymptomatic Type II - symptoms 5-10 days after heparin > 30% drop in platelets venous > arterial thrombosis DVT/PE skin necrosis stroke some may present with acute systemic reactions after IV heparin bolus fever chills dyspnea overt bleeding is rare can see bleeding at injection sites studies Complete blood count drop in platelet count by > 30% no matter the absolute platelet count ↑ bleeding time Normal PT/PTT Presence of anti-PF4-heparin serotonin release assay enzyme immunoassay Differential Diagnosis DIC ITP TTP HUS Treatment Prevention Avoid heparin-containing products First step stop all heparin-containing products Subsequent steps direct thrombin inhibitors argatroban, lepirudin, and bivalirudin transition to warfarin when platelets are back at baseline PLT infusion is contraindicated may worsen thrombosis Complications Thrombosis Prognosis Mortality 5-10% in patients with HIT type II from thrombosis