A clinical prediction rule used to predict the likelihood (pretest probability) of HIT. Includes a consideration of magnitude of platelet count drop, timing of platelet count drop, presence of thrombosis and presence of alternative causes of the reduced platelet count. See calculator.
| Parameter | 2 points | 1 point | 0 points | Comment |
|---|---|---|---|---|
| Thrombocytopenia | Platelet count fall > 50% AND platelet nadir ≥ 20 × 109 L−1 | Platelet count fall 30%–50% OR platelet nadir 10–19 × 109 L−1 | Platelet count fall < 30% OR platelet nadir < 10 × 109 L−1 | Fall from highest platelet count that immediately precedes the putative HIT-related platelet count fall. 95% of cases of HIT are reported to develop in temporal association with heparin therapy; typically > 50% platelet count fall, but not to levels < 20 × 10 9 /L; only a few patients show 30%-50% platelet count fall; typical nadir is 40-80 × 109/L, with median of 55 × 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 | Day 5 to 10 for initial platelet count fall with day 0 representing first heparin exposure; earlier fall if patient exposed to heparin with previous 30 days. Days are rounded off. For example, day 4.3 would count as day 4. |
| 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 |
Scoring 0, 1, or 2 points for each of 4 categories, maximum possible score = 8 Low score (0-3 points), intermediate score (4 or 5 points), high score (6-8 points).
- Low score 0-3 points
- Intermediate score 4-5 points
- High score 6-8 points
Low 4Ts score may rule out HIT but high 4Ts score may not be sufficient to diagnose HIT.