The 4Ts scoring system
| 4Ts category . | 2 points . | 1 point . | 0 points . |
|---|---|---|---|
| Thrombocytopenia | Platelet count fall > 50% and platelet nadir ≥ 20 | Platelet count 30%-50% or platelet nadir 10-19 | Platelet count fall < 30% or platelet nadir < 10 |
| Timing of platelet count fall | Clear onset days 5-10 or platelet fall ≤ 1 day (prior heparin exposure within 30 days) | Consistent with days 5-10 fall, but not clear (eg, missing platelet counts); onset after day 10; or fall ≤ 1 day (prior heparin exposure 30-100 days ago) | Platelet count ≤ 4 days without recent exposure |
| Thrombosis or other sequelae | New thrombosis (confirmed); skin necrosis; acute systemic reaction postintravenous unfractionated heparin bolus | Progressive or recurrent thrombosis; non-necrotizing (erythematous) skin lesions; suspected thrombosis (not proven) | None |
| Other causes of thrombocytopenia | None apparent | Possible | Definite |
| 4Ts category . | 2 points . | 1 point . | 0 points . |
|---|---|---|---|
| Thrombocytopenia | Platelet count fall > 50% and platelet nadir ≥ 20 | Platelet count 30%-50% or platelet nadir 10-19 | Platelet count fall < 30% or platelet nadir < 10 |
| Timing of platelet count fall | Clear onset days 5-10 or platelet fall ≤ 1 day (prior heparin exposure within 30 days) | Consistent with days 5-10 fall, but not clear (eg, missing platelet counts); onset after day 10; or fall ≤ 1 day (prior heparin exposure 30-100 days ago) | Platelet count ≤ 4 days without recent exposure |
| Thrombosis or other sequelae | New thrombosis (confirmed); skin necrosis; acute systemic reaction postintravenous unfractionated heparin bolus | Progressive or recurrent thrombosis; non-necrotizing (erythematous) skin lesions; suspected thrombosis (not proven) | None |
| Other causes of thrombocytopenia | None apparent | Possible | Definite |
The 4Ts score is the sum of the values for each of the 4 categories. Scores of 1-3, 4-5, and 6-8 are considered to correspond to a low, intermediate, and high probability of HIT, respectively.