Thrombocytopenia — Degree of Platelet Count Fall

> 50% fall from baseline with platelet nadir ≥ 20 × 10⁹/L+230–50% fall OR nadir of 10–19 × 10⁹/L+1< 30% fall OR platelet nadir < 10 × 10⁹/L+0

Timing of Platelet Count Fall

Clear onset day 5–10 of heparin; or ≤ 1 day with heparin exposure within the prior 5–30 days (rapid-onset HIT)+2Consistent with days 5–10 fall but unclear onset; or > 10 days after heparin; or ≤ 1 day with heparin exposure 31–100 days prior+1Platelet fall ≤ 4 days without recent prior heparin exposure+0

Thrombosis or Other HIT Sequelae

Confirmed new thrombosis (DVT, PE, stroke, MI); skin necrosis at heparin injection sites; anaphylactic reaction after IV heparin bolus+2Progressive or recurrent thrombosis while on heparin; erythematous skin lesions at heparin injection sites; suspected but not yet confirmed thrombosis+1None+0

oTher Causes for Thrombocytopenia

No other cause apparent — HIT is most likely explanation+2Possible other cause — could be HIT OR another condition+1Definite other cause identified — adequately explains thrombocytopenia+0

4Ts Score for Heparin-Induced Thrombocytopenia (HIT)

4Ts Score for Heparin-Induced Thrombocytopenia (HIT)