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

Updated: Sep 10, 2021
  • Author: Sancar Eke, MD, FASN; Chief Editor: Srikanth Nagalla, MD, MS, FACP  more...
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Diagnosis of heparin-induced thrombocytopenia (HIT) can be challenging, and both underdiagnosis and overdiagnosis pose potential dangers. Failure to diagnose HIT increases the risk of thrombosis, amputation, or death, while misdiagnosis can result in major hemorrhage (in thrombocytopenic patients treated with alternative anticoagulants) or thrombosis (if heparin treatment is suspended unnecessarily). [42] Consequently, pretest scoring systems have been devised to guide diagnosis of HIT.

4Ts score

The 4Ts is a pretest clinical scoring system for HIT that is widely used in clinical practice. [42] 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 [40, 44] (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 at heparin injection sites; anaphylactoid reaction after IV heparin bolus; adrenal hemorrhage

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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