Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) occurs in 2 forms: 

  • HIT type I – also known as heparin-associated thrombocytopenia
    • Non-immune 
    • Occurs in first 48-72 hours after starting heparin
    • Resolves spontaneously on discontinuation of heparin 
    • Not associated with increased risk of thrombosis 
    • Affects up to 10% of patients 
  • HIT type II – heparin-induced thrombocytopenia 
    • Immune-mediated 
    • Occurs 5 to 14 days after starting heparin 
    • Associated with increased risk of thrombosis (1/3) and is potentially life-threatening 
    • Affects around 1-5% of patients on heparin 

When HIT is discussed, physicians usually mean type II. 

Type II HIT is caused by the formation of IgG antibodies against the heparin-platelet factor 4 complex.

Complications associated with HIT: 
  • Thrombosis – occurs in 30-50%, mortality of around 20% 
  • Necrotizing skin lesions at injection site – around 10% 
  • Systemic symptoms – fever, hypertension, tachycardia – up to 25% 

There is a higher risk of HIT with unfractionated heparin compared to LMWH 

Treatment of HIT 
  • Discontinue heparin 
  • Initiative alternative anticoagulation 
    • Lepirudin 
      • a direct, irreversible thrombin inhibitor 
      • requires aPTT monitoring every 4 hours until a steady state is achieved 
      • risk of bleeding
      • risk of anaphylaxis if second course of treatment 
    • Danaparoid 
      •  inhibits factor Xa and to a lesser degree thrombin 
      • less bleeding risk
    • Argatroban
      • direct thrombin inhibitor
      • metabolised by the liver
      • shortest half life of the 3 alternatives
      • less bleeding risk than lepirudin
  • Do not give warfarin until platelet count has recovered. 
  • Reverse warfarin if already given 
  • Avoid platelet transfusion if possible as this may exacerbate the hypercoagulable state 

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