Canterbury DHB
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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 |
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Low PTP |
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 |
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Item |
Point Score |
I. Thrombocytopenia |
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Platelet count fall >50 percent and nadir >20,000 |
2 |
Platelet count fall 30-50 percent or nadir 10-19,000 |
1 |
Platelet count fall <30 percent or nadir <10,000 |
0 |
II. Timing of platelet count fall |
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Clear onset between days 5-10 or platelet count fall ≤1 day if prior heparin exposure within the last 30 days |
2 |
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 |
1 |
Platelet count fall at <4 days without recent exposure |
0 |
III. Thrombosis or other sequelae |
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Confirmed new thrombosis, skin necrosis, or acute systemic reaction post-IV unfractionated heparin bolus |
2 |
Progressive or recurrent thrombosis, non-necrotizing (erythematous) skin lesions, or suspected thrombosis which has not been proven |
1 |
None |
0 |
IV. Other causes for thrombocytopenia present |
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None apparent |
2 |
Possible |
1 |
Definite |
0 |
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:
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Topic Code: 5190
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