Heparin induced thrombocytopenia (HIT)


Heparin induced thrombocytopenia (HIT) can be a serious complication of unfractionated heparin therapy. There are two types of HIT:

Type I HIT: This form is non-immune mediated. Thrombocytopenia (low platelet count) is seen within the first 48 hours of therapy and will normalize with continued therapy. There is no serious adverse risk such as thrombosis.

Type II HIT: Also known as “heparin induced thrombocytopenia and thrombosis” or HITT, this type is due to an autoimmune reaction causing increased platelet activation. Thrombocytopenia is seen within 5-7 days of therapy. HIT creates a hypercoagulable state increasing the risk of thrombosis (the opposite of the desired effect of heparin) and.


The four Ts score is frequently used to confirm the diagnosis of type II HIT or HITT. A score of 0-3 indicates low likelihood, 4-5 intermediate and 6-8 high likelihood.


2 points = Platelets decreased 50% from original level or decrease to between 20-100 K/µL

1 point = Platelets decreased 30-50% from original level or to 10-19 K/µL

0 points = Platelets decrease < 30% or to < 10 K/µL


2 points = Decreased platelets 5-10 days into treatment*

1 point = Decreased platelets > 10 days into treatment*


2 points = Confirmed thrombosis or skin necrosis

1 point = Progressive recurrent thrombosis

Alternative explanation

2 points = No other cause present

1 point = Possible other cause

0 points = Definite alternative cause

*If prior heparin exposure within 30 days is present, a decrease can occur more rapidly. If it occurs in < 24 hours, 2 points is given. If prior exposure was within 30-100 days and a decrease occurs in < 24 hours, 1 point is given.

Diagnosis of type II HIT can also be made using two serum tests: Heparin induced platelet aggregation assay or HIPA (positive if HIT is present) and serotonin release assay or SRA (positive if HIT present).


Treatment of type II HIT includes the discontinuation of heparin and the use of a direct thrombin inhibitor as alternate anticoagulation such as argatroban or lepirudin. No treatment is indicated for type I HIT.


Warfarin (Coumadin) should be used in the setting of type II HIT until the platelet count increases to at least 150 K/µL due to the risk of Coumadin necrosis.