Table 1.

4T Predictive Scoring System for Thrombocytopenia

4T’s	2 Points	1 Point	0 Point
Thrombocytopenia	>50% decrease in platelet count and platelet nadir of >20 x 109/L No surgery within the preceding 3 days	>50% fall in platelet count but surgery within preceding 3 days OR Any combination of platelet fall and platelet nadir that does not meet criteria for a score of 2 points	<30% platelet fall OR Any platelet nadir <10 x 109/L
Timing of platelet count	Platelet count falls 5–10 days after start of heparin therapy OR Platelets fall within 1 day of start of heparin with exposure to heparin within the past 5–30 days	Onset unclear OR Platelet fall after day 10 OR Platelet fall within 1 day of starting heparin (if exposed to heparin within the past 30–100 days)	Platelet fall within 4 days without exposure to heparin in the past 100 days
Thrombosis	Confirmed new thrombosis OR Skin necrosis at injection site OR Anaphylactic reaction to IV heparin bolus.	Progressive or recurrent venous thrombosis OR Suspected thrombosis OR Red skin lesions at heparin injection site	Thrombosis not suspected
Other causes of thrombocytopenia	No other cause is present	Possible other cause is present	Probable other cause is present

Notes: Interpretation: ≤3 points: low probability for HIT. 4–5 points: intermediate probability (around 14% probability of HIT). 6–8 points: high probability (around 64% probability of HIT). Data from Lo et al.³⁴