Canterbury DHB


Heparin-induced Thrombocytopenia (HIT)

This should be considered in all patients who become thrombocytopenic (or drop their platelet count by 50% within normal range) while on heparin therapy. The incidence is higher in patients receiving unfractionated heparin but should also be considered in patients on low-molecular weight heparin.

If patients have a recent history of HIT or are suspected of HIT, laboratory testing should be carried out and treatment considered following a positive result from the test. The low incidence of HIT is associated with a significant false positive rate therefore the test result should be considered in the context of the pre-test probability.

The pre-test scoring system using the “4T’s” (developed by Lo, Juhl, Warkentin, et al) gives a low, intermediate or high pre-test probability (PTP) which will assist in the diagnostic process. See table below for assessment of 4Ts.

Interpretation of results


HIT unlikely, no testing / treatment required

Intermediate PTP with negative test

Intermediate PTP with positive test

High PTP with test negative

HIT possible, consider further investigation and treatment following discussion with Haematologist

High PTP with positive test

HIT likely, start treatment

For further information regarding HIT, see Watson, H., et al. (2012) 'Guidelines on the diagnosis and management of heparin-induced thrombocytopenia: Second edition.' BJH 159(5): 528-540.

Pretest scoring system for heparin-induced thrombocytopenia


Point Score

I. Thrombocytopenia

Platelet count fall >50 percent and nadir >20,000


Platelet count fall 30-50 percent or nadir 10-19,000


Platelet count fall <30 percent or nadir <10,000


II. Timing of platelet count fall

Clear onset between days 5-10 or platelet count fall ≤1 day if prior heparin exposure within the last 30 days


Consistent with fall at 5-10 days but not clear (eg, missing platelet counts) or onset after day 10 or fall ≤1 day with prior heparin exposure within the last 30-100 days


Platelet count fall at <4 days without recent exposure


III. Thrombosis or other sequelae

Confirmed new thrombosis, skin necrosis, or acute systemic reaction post-IV unfractionated heparin bolus


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




IV. Other causes for thrombocytopenia present

None apparent






How to use this scoring system: Determine a score (from 0 to 2) for each of the above categories, resulting in a total potential score from zero to eight. The pretest probabilities for the presence of heparin-induced thrombocytopenia based on this scoring system are, as follows:

Reference: Lo, GK, Juhl, D, Warkentin, TE, et al. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006; 4:759.

In This Section

Management of HIT

About this Canterbury DHB document (5190):

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Last Reviewed:

August 2016

Next Review:

August 2018


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Topic Code: 5190