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. Author manuscript; available in PMC: 2014 Sep 3.
Published in final edited form as: Hematology Am Soc Hematol Educ Program. 2013;2013:668–674. doi: 10.1182/asheducation-2013.1.668

Table 1.

4Ts scoring system

Category 2 points 1 point 0 points
(a) 4 T’s
1. Thrombocytopenia Platelet count fall > 50% and platelet nadir ≥ 20 × 109 L−1 Platelet count fall 30%–50% or platelet nadir 10–19 × 109 L−1 Platelet count fall < 30% or platelet nadir < 10 × 109 L−1
2. Timing of platelet count fall Clear onset between days 5 and 10 or platelet fall ≤ 1 day (prior heparin exposure within 30 days) Consistent with days 5–10 fall, but not clear (e.g. missing platelet counts) or onset after day 10 or fall ≤ 1 day (prior heparin exposure 30–100 days ago) Platelet count fall < 4 days without recent heparin exposure
3. Thrombosis or other sequelae New thrombosis (confirmed) or skin necrosis at heparin injection sites or acute systemic reaction after intravenous heparin bolus Progressive or recurrent thrombosis or nonnecrotizing (erythematous) skin lesions or suspected thrombosis (not proven) None
4. Other causes for thrombocytopenia None apparent Possible Definite

Total score of ≥ 6 points, high probability of HIT; 4–5 points, intermediate probability of HIT; ≤ 3 points, low probability of HIT. CPB, cardiopulmonary bypass.