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
If (+) HIT – AVOID HEPARIN FOR LIFE!