Diagnosis of Heparin-Induced Thrombocytopenia (HIT): the 4t Score
| Category | 2 points | 1 point | 0 points |
|---|---|---|---|
| Thrombocytopenia | Platelet count fall >50% and platelet nadir ≥20×109/L | Platelet count fall 30–50% or platelet nadir 10–19×109/L | Platelet count fall <30% or platelet nadir <10×109/L |
| 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 |
| 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 non-necrotizing (erythematous) skin lesions or suspected thrombosis (not proven) | None |
| Other causes for thrombocytopenia | None apparent | Possible | Definite |
Total points:
≤3 = low probability of HIT
4–5 = intermediate probability of HIT
≥6 = high probability of HIT
Source: Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A (2006) Evaluation of pretest clinical score (4 Ts) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost 4, 759–65. Reproduced with permission of John Wiley & Sons.