More than 12 million Americans have been treated withheparin annually．Heparin-inducedthrombocytopenia(HIT)has become one of the mostdevastating and serious immune-mediated adversedrug reactions in current clinical practice.In theUnited States alone，the annual estimated health carecost of HIT associated complications in cardiacsurgery is about $300 million．HIT is the development of thrombocytopenia secondary toadministration of heparin，typically 5 to 14 daysafterexposure．Low-molecular weight heparin(LMWH)Can alsocause HIT，but at significantly lower rates．Despite of the low platelet count,HIT is aprothrombotic disorder presented with thrombosis inthevenous and arterial microvasculature，which can leadto catastrophic sequela such as limb gangrene or life-threatening thromboembolic complications．HIT exists in two forms．Type Ⅰ HIT is acommon，non-immunologic phenomenon occurring in up to 20％ of all patients involving a transientdecrease in platelets following administrationofheparin．Type Ⅰ HIT resolves even with thecontinuedadministration of heparin．
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