4Ts Score for Heparin-Induced Thrombocytopenia (HIT)

Estimates pretest probability of HIT from thrombocytopenia, timing, thrombosis, and alternative causes.

Thrombocytopenia*

Platelet count fall >50% AND platelet nadir ≥20 × 10^9/L+2

Platelet count fall 30%–50% OR platelet nadir 10–19 × 10^9/L+1

Platelet count fall <30% OR platelet nadir <10 × 10^9/L0

Timing of platelet count fall*

Clear onset between days 5–10 OR platelet fall ≤1 day (prior heparin exposure within 30 days)+2

Consistent with days 5–10 fall, but not clear; onset after day 10 OR fall ≤1 day (prior heparin exposure 30–100 days ago)+1

Platelet count fall <4 days without recent heparin exposure0

Thrombosis or other sequelae*

New thrombosis OR skin necrosis; acute systemic reaction post-IV heparin bolus+2

Progressive OR recurrent thrombosis; non-necrotizing skin lesions; suspected thrombosis (not proven)+1

None0

Other causes for thrombocytopenia*

None apparent+2

Possible+1

Definite0