HEPARIN INDUCED THROMBOCYTOPENIA (HIT)
Heparin induced thrombocytopenia follows exposure to unfractionated heparin, but also, but less commonly low molecular weight heparin, LMWH.
Low platelet count below 150,000/mm3 or a relative decrease of greater than or equal to 50% from baseline. The fall may be less, such as 30 to 40% in some patients. It is very rare to fall to less than 10,000. Thrombotic (clot) complications, such as strokes and heart attacks, in 20 to 50% of patients. This is known to be 30 times the rate in control populations. The risk remains even after the platelet count recovers. Recover occurs usually in 4 to 14 days after discontinuing the heparin.
In HIT, there is the formation of antibodies against a complex formed from PF4 and heparin. This is evident in nearly all patients with the clinical diagnosis of this disorder, and this is the basis for the Heparin Induced Thrombocytopenia Assay which can detect this diagnosis. However, due to the low specificity of this assay, it is crucial to send off a Serotonin Release Assay (SRA) in order to truly confirm the diagnosis of Heparin Induced Thrombocytopenia. This assay detects the lysis of platelets when antibodies attach to them, releasing the intracellular serotonin, making the test very specific.
The onset of thrombocytopenia, (low platelets) is 5 to 10 days with no previous exposure or remote greater than 100 days in exposure. There is a precipitous decline in platelets within hours of recent exposure and detectable levels of heparin-PF4 complex antibodies.
1) Warkentin, T.E. Heparin-Induced Thrombocytopenia: Pathogenesis and Management. British J. Haematology 2003, 121, 535-555.