Heparin-Induced Thrombocytopenia – Stuart

HIT occurs in 5% of patients exposed to heparin.

Etiology:  Autoantibody to PF4-heparin complex.

More common with UFH, in women, and with surgery.

Thrombocytopenia is noted in 90%, thrombosis in 50%.

Dx:  Lab:  HIT antibody testing:  PF4 antibody, IgG

Clinical Dx:  Four T Score:

4 T’s score

[From UpToDate, 4/21/17]:  Calculating the score — The 4 T’s score is used for estimating the likelihood of HIT based on readily available clinical features (calculator 1) [97,98]. It is used to make a presumptive diagnosis of HIT until laboratory data are available, and then integrated with laboratory data to make a final diagnosis.

The score assesses the degree of Thrombocytopenia, the Timing relative to heparin exposure, the presence of Thrombosis, and oTher causes for thrombocytopenia. Points are assigned as follows:


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 points

Platelet count fall <30 percent or nadir <10,000/microL – 0 points

Timing of platelet count fall

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

Consistent with fall at 5 to 10 days but unclear (eg, missing platelet counts), 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 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

None apparent – 2 points

Possible – 1 point

Definite – 0 points


Score each one 0, 1 or 2.

Total score:  0-3 = low risk, 4-5 = intermediate risk, 6-8 = high risk



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