4Ts Score for Heparin-Induced Thrombocytopenia

Differentiates patients with HIT from those with other causes of thrombocytopenia.

Thrombocytopenia

Platelet count fall >50% and platelet nadir ≥20

Platelet count fall 30-50% or platelet nadir 10-19

Platelet count fall <30% or platelet nadir <10

Timing of Platelet Count Fall

Clear onset between days 5-10 or platelet fall ≥1 day (prior heparin exposure within 30 days)

Consistent with days 5-10 fall, but not clear; onset after day 10; or fall ≥1 day (prior heparin exposure 30-100 days ago)

Platelet count fall <4 days without recent exposure

Thrombosis or other sequelae

New thrombosis or skin necrosis; acute systemic reaction post-IV heparin bolus

Progressive or recurrent thrombosis; non-necrotizing skin lesions; suspected thrombosis (not proven)

None

Other causes for thrombocytopenia

None apparent

Possible

Definite

Interpretation

Source: Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 Ts) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006; 4: 759–65.

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