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The 4T's

Estimating the pretest probability of HIT based on clinical features (adapted from Lo et al. 2006)

2 point
1 point
0 point


>50% platelet fall
(nadir ? 20 x 109/L )   

30-50% platelet fall (or >50% fall due to surgery); or nadir 10-20 x 109/L <30% platelet fall; or nadir
? 10 x 109/L

Timing of platelet count fall
(First day of heparin course =day 0)   

 Clear onset between days 5-10 or ? 1 day (if heparin exposure within 30 days) 

Consistent with day 5-10 fall, but not clear (e.g. missing platelet counts)
or  ? 1 day (prior heparin exposure within 30-100 days) or onset after day 10    

Platelet count fall < 4 days without recent heparin exposure

Thrombosis or other sequelae (e.g.skin lesion, acute systemic reactions) 

New thrombosis; skin necrosis; acute systemic reaction following iv heparin bolus   

Progressive or recurrent thrombosis; erythematous skin lesions; suspected thrombosis (not yet proven); asymptomatic upper-limb DVT   



No explanation (besides HIT) for platelet count fall is evident

Possible other cause is evident 

Definite other cause is present

Pretest probability score
Score of 6-8 points = high risk of HIT
Score of 4-5 points = moderate risk
Score of 0-3 points = low risk of HIT

1. Lo GK, Juhl D, Warkentin TE et al. Evaluation of pretest clinical score (4T’s) for the diagnosis of heparin–induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006; 4:759-65. 

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After cardiac surgery