4Ts Score (HIT)

Select the single best-matching option in each section (highest applicable category within that domain).

T — Thrombocytopenia (platelet count fall and nadir)

2 pts — Fall >50% and nadir ≥20×10⁹/L (≥20,000/µL) 1 pt — Fall 30–50% OR nadir 10–19×10⁹/L 0 pts — Fall <30% OR nadir <10×10⁹/L

T — Timing of platelet count fall (relative to heparin)

2 pts — Clear onset between days 5–10; OR fall ≤1 day with heparin exposure within prior 30 days 1 pt — Consistent with days 5–10 but not clear; OR onset after day 10; OR fall ≤1 day with heparin 30–100 days ago 0 pts — Platelet fall ≤4 days without recent heparin exposure

T — Thrombosis or other clinical sequelae

2 pts — Confirmed new thrombosis, skin necrosis at heparin injection sites, or anaphylactoid reaction after IV bolus 1 pt — Progressive/recurrent thrombosis, non-necrotizing skin lesions, or suspected (not proven) thrombosis 0 pts — None

oTher — Alternative causes of thrombocytopenia

2 pts — No apparent alternative cause 1 pt — Possible alternative cause 0 pts — Definite alternative cause