Criteria for estimating pre-test probability of HIT using the “4 T’” score (from Warkentin7 and Bryant et al26). The pre-test probability score would be high for a score of 6–8, intermediate for a score of 4–5, and low for a score of 0–3.
| . | Points (0, 1, or 2 for each of 4 categories; maximal possible score = 8) . | ||
|---|---|---|---|
| Findings . | 2 . | 1 . | 0 . |
| Thrombocytopenia | >50% fall or platelet nadir 20–100 × 109/L | 30%–50% fall or platelet nadir 10–19 × 109/L | Fall <30% or platelet nadir < 10 × 109/L |
| Timing of platelet count fall or other sequelae | Clear onset between day 5–10; or less than 1 day (if heparin exposure within past 30 days). | Consistent with immunization but not clear, or onset of thrombocytopenia after day 10, or within 1 day and prior heparin exposure within past 31–100 days. | Platelet count falls too early, without recent heparin exposure. |
| Thrombosis of other sequelae (eg, skin lesions) | New thrombosis; skin necrosis post heparin bolus acute systemic reaction. | Progressive or recurrent thrombosis; erythematous skin lesions; clinically suspected thrombosis not yet proven. | None. |
| Other cause for thrombocytopenia not evident | No other cause for platelet count fall is evident. | Possible other cause is evident. | Definite other cause is present. |
| . | Points (0, 1, or 2 for each of 4 categories; maximal possible score = 8) . | ||
|---|---|---|---|
| Findings . | 2 . | 1 . | 0 . |
| Thrombocytopenia | >50% fall or platelet nadir 20–100 × 109/L | 30%–50% fall or platelet nadir 10–19 × 109/L | Fall <30% or platelet nadir < 10 × 109/L |
| Timing of platelet count fall or other sequelae | Clear onset between day 5–10; or less than 1 day (if heparin exposure within past 30 days). | Consistent with immunization but not clear, or onset of thrombocytopenia after day 10, or within 1 day and prior heparin exposure within past 31–100 days. | Platelet count falls too early, without recent heparin exposure. |
| Thrombosis of other sequelae (eg, skin lesions) | New thrombosis; skin necrosis post heparin bolus acute systemic reaction. | Progressive or recurrent thrombosis; erythematous skin lesions; clinically suspected thrombosis not yet proven. | None. |
| Other cause for thrombocytopenia not evident | No other cause for platelet count fall is evident. | Possible other cause is evident. | Definite other cause is present. |