Heparin Induced Thrombocytopenia Pre-Test Scoring System

1. 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
2. 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)
3. References
   1. Lo (2006) J Thromb Haemost 4: 759–65