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Calculator: 4 Ts score for calculating the pretest probability of heparin-induced thrombocytopenia (HIT)

Calculator: 4 Ts score for calculating the pretest probability of heparin-induced thrombocytopenia (HIT)


Platelet count fall >50% AND nadir ≥20,000/microL (2 points)

Platelet count fall 30 to 50% OR nadir 10,000 to 19,000/microL (1 point)

Platelet count fall <30% OR nadir <10,000/microL (0 points)
Timing of platelet count fall

Clear onset between days 5 and 10 of heparin exposure OR platelet count fall at ≤1 day if prior heparin exposure within the last 30 days (2 points)

Consistent with fall in platelet count at 5 to 10 days but unclear (eg, missing platelet counts) OR onset after day 10 OR fall ≤1 day with prior heparin exposure within 30 to 100 days (1 point)

Platelet count fall at <4 days without recent heparin exposure (0 points)
Thrombosis or other sequelae

Confirmed new thrombosis, skin necrosis, or acute systemic reaction after intravenous unfractionated heparin bolus (2 points)

Progressive or recurrent thrombosis, non-necrotizing (erythematous) skin lesions, or suspected thrombosis that has not been proven (1 point)

None (0 points)
Other causes for thrombocytopenia present

None apparent (2 points)

Possible (1 point)

Definite (0 points)

Important: Inputs must be complete to perform calculation.

Total criteria point count:


4 Ts score interpretation

0 to 3 points: Low probability
4 to 5 points: Intermediate probability
6 to 8 points: High probability

  • This score has not been well validated in all populations. Limited pediatric data are available. Refer to UpToDate for additional details of our approach to incorporating the 4 Ts score when evaluating for the likelihood of HIT.

Only digits 0 to 9 and a single decimal point (".") are acceptable as numeric inputs. Attempted input of other characters into a numeric field may lead to an incorrect result.

Information on this page may not appear correctly if printed.

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