Heparin-induced thrombocytopenia (HIT)

Regarding heparin-induced thrombocytopenia (HIT) the following statements are true

EXCEPT which one?

a) The patient receiving low molecular weight heparin is less likely to develop HIT

than the patient receiving unfractionated heparin

b) A diagnosis of HIT is more likely if the platelet count falls to 50 x109/L than falls to

10 x109/L

c) The assays used to make a diagnosis of HIT have a higher specificity than they do

sensitivity

d) A patient with HIT is more likely to develop thrombosis than a similar patient

without HIT

e) Prophylactic platelet transfusions should be avoided in a patient with HIT

Answer: c

Explanation

Heparin-induced thrombocytopenia is defined as a decrease in platelet count during or

shortly after exposure to heparin. There are two types of HIT. HIT type 1 affects up to

10% of patients receiving treatment with heparin, is non-immune, is characterised by a

mild and transient asymptomatic thrombocytopenia and disappears when the heparin

is withdrawn. HIT type 2 is immune mediated and associated with thrombosis. The

latter occurs in 1 to 5% of patients receiving unfractionated heparin. The risk is much

lower with low molecular weight heparin. The increased thrombosis risk is due to the

development of an antigen complex of heparin and platelet factor 4 (PF4). This leads

to increased platelet activation thus predisposing to thrombosis. As the assays used to

diagnose HIT are extremely sensitive but not specific it is important to limit testing to

those patients deemed at highest risk of HIT type 2. Various HIT scoring systems exist

to determine those patients who should be tested. These are based around the ‘4Ts’ –

Thrombocytopenia, Timing, Thrombosis and the absence of oTher explanations.

Mortality in HIT type 2 is high and treatment other than general supportive care starts

with immediate cessation of heparin administration. Danaparoid, epoprostenol,

lepirudin or argatroban may be used as alternative anticoagulants. There may be

possible treatment benefit with plasmapheresis, aspirin, clopidogrel and glycoprotein

IIb/IIIa inhibitors. Platelet transfusion and warfarin should both be avoided.

Reference

Arepally G, Ortel TL. Clinical practice. Heparin-induced thrombocytopenia. New Engl J

Med 2006; 355(8): 809–17.