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
