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Table 2 The 4 T's score [32]

From: Heparin-induced thrombocytopenia in the ICU: an overview

4 T Category 2 1 0
Thrombocytopenia Platelet count decrease > 50% or platelet nadir ≥ 20 × 109/L Platelet count decrease 30-50% or platelet nadir 10-19 × 109/L Platelet count decrease < 30% or platelet nadir < 10 × 109/L
Time to platelet count decrease* Clear onset between days 5-10 or platelet count decrease ≤ 1 day (prior heparin exposure within 100 days) Consistent with decrease between days 5-10, but not clear (e.g., missing platelet counts); onset after day 10 Platelet count decrease < 4 days without recent exposure
Thrombosis or other sequelae New thrombosis (confirmed); skin necrosis; acute systemic reaction postintravenous heparin Progressive or recurrent thrombosis; non-necrotizing (erythematous) skin lesions; suspected thrombosis (not proven) None
Other causes of thrombocytopenia None apparent Possible Definite
  1. *First day immunizing heparin exposure considered day zero; the day the platelet count begins to decrease is considered the day of onset of thrombocytopenia.