The 4Ts score sums four clinical domains, each scored 0–2 points:

Domain	2 points	1 point	0 points
Thrombocytopenia	Platelet count fall >50% AND nadir ≥20 × 10^9/L.	Platelet count fall 30–50% OR nadir 10–19 × 10^9/L.	Platelet count fall <30% OR nadir <10 × 10^9/L.
Timing of platelet fall	Clear onset between days 5–10 OR fall ≤1 day with prior heparin exposure within the past 30 days.	Onset compatible with days 5–10 but not clear (e.g. missing counts), OR after day 10, OR fall ≤1 day with heparin exposure 30–100 days ago.	Platelet fall within 4 days without recent heparin exposure.
Thrombosis or other sequelae	New confirmed thrombosis, skin necrosis, acute systemic reaction after intravenous heparin bolus, or adrenal haemorrhage.	Progressive or recurrent thrombosis, suspected (unproven) thrombosis, or non-necrotising erythematous skin lesions at heparin sites.	None.
Other causes of thrombocytopenia	No other apparent cause.	Possible other cause.	Definite other cause present.

Total score is the sum of all domains (range 0–8 points); higher scores indicate higher pre-test probability of HIT.

4Ts Score for Heparin-Induced Thrombocytopenia