Table 1.

Pre-test probability of HIT: ‘the four T’s’3

Category		Score
	2	1	0
Thrombocytopenia	50% fall or platelet nadir 20–100 × 109/l	30–50% fall or platelet nadir 10–19 × 109/l	Fall < 30% or platelet nadir < 10 × 109/l
Timing* of platelet count fall or other sequelae	Clear onset between Days 5–10; or less than 1 day (if heparin exposure within past 100 days) )	Consistent with immunisation but not clear (e.g. missing platelet counts or onset of thrombocytopenia after day 10	Platelet count fall too early (without recent heparin exposure)
Thrombosis or other sequelae (e.g. skin lesions)	New thrombosis; skin necrosis; post-heparin bolus acute systemic reaction	Progressive or recurrent thrombosis; erythematous skin lesions; suspected thrombosis not yet proven	None
Other causes for thrombocytopenia not evident	No other cause for platelet count fall is evident	Possible other cause is evident	Definite other cause is present

Pre-test probability score: 6–8, high; 4–5, intermediate; 0–3, low.

^*First day of immunising heparin exposure considered Day 0; the day the platelet count begins to fall is considered the day of onset of thrombocytopenia (it generally takes 1–3 days more until an arbitrary threshold that defines thrombocytopenia is passed).