4T score HIT clinical scoring system
| . | 2 points . | 1 point . | 0 points . |
|---|---|---|---|
| Thrombocytopenia | Platelet count fall >50% and platelet count nadir ≥20 × 109/L | Platelet count fall 30-50% or platelet count nadir 10-19 × 109/L | Platelet count fall <30% or platelet count nadir < 10 × 109/L |
| Timing of platelet count fall | Clear onset of thrombocytopenia 5-10 d after heparin administration or platelet count fall ≤1 d with prior heparin exposure within 30 d | Timing is consistent with a day 5-10 platelet count fall but is not clear (eg, missing platelet counts), onset after day 10, or fall ≤1 d with prior heparin exposure 30-100 d ago | Platelet count fall is <4 d without recent heparin exposure |
| Thrombosis or other sequelae | Confirmed evidence of a new thrombosis, skin necrosis (lesions at heparin injection site, or acute systemic reaction after IV unfractionated heparin bolus | Suspected (but not proven) thrombosis, progressive or recurrent thrombosis, or nonnecrotizing skin lesions | None |
| Other causes of thrombocytopenia | None apparent | Possible | Definite |
| . | 2 points . | 1 point . | 0 points . |
|---|---|---|---|
| Thrombocytopenia | Platelet count fall >50% and platelet count nadir ≥20 × 109/L | Platelet count fall 30-50% or platelet count nadir 10-19 × 109/L | Platelet count fall <30% or platelet count nadir < 10 × 109/L |
| Timing of platelet count fall | Clear onset of thrombocytopenia 5-10 d after heparin administration or platelet count fall ≤1 d with prior heparin exposure within 30 d | Timing is consistent with a day 5-10 platelet count fall but is not clear (eg, missing platelet counts), onset after day 10, or fall ≤1 d with prior heparin exposure 30-100 d ago | Platelet count fall is <4 d without recent heparin exposure |
| Thrombosis or other sequelae | Confirmed evidence of a new thrombosis, skin necrosis (lesions at heparin injection site, or acute systemic reaction after IV unfractionated heparin bolus | Suspected (but not proven) thrombosis, progressive or recurrent thrombosis, or nonnecrotizing skin lesions | None |
| Other causes of thrombocytopenia | None apparent | Possible | Definite |
The scoring system is categorized as low probability (0-3 points), intermediate probability (4-5 points), or high probability (6-8 points). Adapted from Lo et al59 with permission.