What is the 4Ts pretest score for heparin-induced thrombocytopenia (HIT) and how is it calculated?

Updated: Apr 24, 2018
  • Author: Sancar Eke, MD, FASN; Chief Editor: Srikanth Nagalla, MBBS, MS, FACP  more...
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The 4Ts is a pretest clinical scoring system for HIT that is widely used in clinical practice. [38] See the Table below. The title refers to four characteristics of HIT:

  • Thrombocytopenia

  • Timing of thrombocytopenia relative to heparin exposure

  • Thrombosis or other sequelae of HIT

  • Likelihood of other (oTher) causes of thrombocytopenia

Table. 4Ts score [36, 39] (Open Table in a new window)



2 points

1 point

0 points


>50% fall


platelet nadir 20-100 × 109/L

30%-50% fall


platelet nadir 10-19× 109/L

>30% fall


platelet nadir < 10×109/L

Timing of platelet count fall

Clear onset on day 5-10, or =1 d if heparin exposure within past 30 d

Consistent with day 5-10 fall, but not clear (eg, missing platelet counts); onset after day 10; or fall = 1 day if heparin exposure 30-100 days ago

Platelet count fall =4 d without recent heparin exposure

Thrombosis or other sequelae

New thrombosis (confirmed); skin necrosis; acute systemic reaction after IV UHF bolus

Progressive or recurrent thrombosis; erythematous skin lesions; thrombosis suspected but not proven


Other causes of thrombocytopenia

None apparent



Total scores and corresponding probability of HIT are as follows:

  • 0-3: Low probability

  • 4-5: Intermediate probability

  • 6-8: High probability

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