The 4Ts scoring system to evaluate the pretest risk for HIT
| 4Ts . | 2 points . | 1 point . | 0 point . |
|---|---|---|---|
| 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 d* | Onset after day 10 or fall ≤1 d† | Platelet count fall <4 or >14 d after exposure |
| Thrombosis or other sequelae | New thrombosis‡ (confirmed) | Suspected thrombosis (not proven) | None |
| Other causes for thrombocytopenia | None apparent | Possible | Definite |
| 4Ts . | 2 points . | 1 point . | 0 point . |
|---|---|---|---|
| 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 d* | Onset after day 10 or fall ≤1 d† | Platelet count fall <4 or >14 d after exposure |
| Thrombosis or other sequelae | New thrombosis‡ (confirmed) | Suspected thrombosis (not proven) | None |
| Other causes for thrombocytopenia | None apparent | Possible | Definite |
The resulting clinical probability score is divided into high (6-8 points), intermediate (4-5 points), and low (≤3 points).
In case of prior heparin exposure during the last 30 days.
In case of prior heparin exposure within 30 to 100 days.
Also skin necrosis, acute systemic reaction postintravenous UFH bolus, progressive or recurrent thrombosis, and non-necrotizing (erythematous) skin lesions.