| 4T score | 2 points | 1 point | 0 points |
|---|---|---|---|
| Thrombocytopenia | Platelet count fall >50% and platelet nadir ≥20 000/mm3 | Platelet count fall 30%-50% or platelet nadir 10 000-19 000/mm3 | Platelet count fall <30% or platelet nadir <10 000/mm3 |
| Timing of platelet count fall | Clear onset between 5 and 10 d or platelet fall ≤1 d (prior heparin exposure within 30 d) | Consistent with days 5-10 fall, but not clear (eg, missing platelet counts); onset after day 10 or fall ≤1 d (prior heparin exposure 30-100 d ago) | Platelet count fall <4 d without recent exposure |
| Thrombosis or other sequela | New thrombosis (confirmed); skin necrosis; acute systemic reaction postintravenous UFH bolus | Progressive or recurrent thrombosis; non-necrotizing (erythematous) skin lesions; suspected thrombosis (not proven) | None |
| Other causes for thrombocytopenia | Nonapparent | Possible | Definite |
For patients with a 4T score of <4 points, the probability of having immune HIT is <5%.