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.