How to Calculate HIT 4T Score

HIT 4T score is a rating used to help clinicians

Evaluation of pretest clinical score (4 T’s) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings

G K Lo , D Juhl, T E Warkentin, C S Sigouin, P Eichler, A Greinacher

What is HIT? “Heparin-induced thrombocytopenia (HIT)”

It is a side effect of heparin treatment that causes thrombocytopenia (low platelet count). HIT may be classified into two parts. In most cases, type 1 HIT manifests itself during the first 2 days after being exposed to heparin, and the platelet count returns to normal with continuous heparin treatment.

HIT type 1

Type 1 HIT is a non-immune disease that develops as a consequence of the direct impact of heparin on the activation of platelet cells.

HIT type 2

A type 2 HIT is an immune-mediated disease that usually manifests itself 4-10 days after exposure to heparin and is associated with life- and limb-threatening thrombotic events. Type 2 HIT is more common in women than in men. When the word HIT is used in normal medical practice, it refers to type 2 HIT.

4 t score

How to Calculate HIT 4T Score

HIT 4T Score Calculator

1. Thrombocytopenia

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

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

Platelet count fall <30 percent OR nadir <10,000/microL (0 points)

2. 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)

3. 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)

4. Other causes for thrombocytopenia present

None apparent (2 points)

Possible (1 point)

Definite (0 points)

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HIT criteria

The HIT 4Ts is an algorithm to help in the diagnosis HIT of patients who develop thrombocytopenia in the clinical environment.

Scoring system consisted of four criteria, each of which was worth 0, 1, or 2 points.

Study used ≤ 3 points to define low probability group (≤5%) for HIT, 4-5 points for intermediate and 6-8 points for high.