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Updated: Feb 5 2021

Heparin-Induced Thrombocytopenia (HIT)

  • 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
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