II. Criteria (Mnemonic: 4Ts)

  1. Thrombocytopenia
    1. Score 2: Platelet Count decreased more than 50% from baseline with nadir >20,000
    2. Score 1 Platelet Count decreased 30-50% from baseline with nadir 10-20,000
    3. Score 0: Platelet Count decreased less than 30% from baseline with nadir <10,000
  2. Timing after onset of current Heparin course
    1. Score 2: Clear onset days 5-10 (or <1 day if prior exposure Heparin within last 30 days)
    2. Score 1: Probable onset days 5-10 (or <1 day if prior exposure Heparin 30-100 days ago)
    3. Score 0: Onset within 4 days and no prior Heparin exposure before this episode
  3. Thrombosis
    1. Score 2: Confirmed new clot, skin necrosis and systemic reaction
    2. Score 1: Progressive, recurrent or suspected clot
    3. Score 0: No thrombosis
  4. Thrombocytopenia other causes
    1. Score 2: No other alternative cause of Thrombocytopenia
    2. Score 1: Possible alternative cause of Thrombocytopenia
    3. Score 0: Definite alternative cause of Thrombocytopenia

III. Interpretation: Likelihood of Heparin Induced Thrombocytopenia

  1. Calculate cummulative score of all 4 components
  2. Total Score 0-3: Low probability (less than 5% risk of Clinically Significant HIT)
  3. Total Score 4-5: Intermediate probability
  4. Total Score 6-8: High probability (>80% risk of Clinically Significant HIT)

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