Classic heparin-induced thrombocytopenia

Classic heparin-induced thrombocytopenia (HIT), also known as heparin-induced thrombocytopenia type II (HIT type II), is an acquired, immune-mediated adverse drug reaction that occurs in patients exposed to heparin therapy. It is characterized by the formation of antibodies (typically IgG) directed against complexes of platelet factor 4 (PF4) and heparin. These antibodies activate platelets, leading to a paradoxical prothrombotic state despite the presence of thrombocytopenia (low platelet count). HIT primarily affects the hematologic and vascular systems. The hallmark clinical feature is a significant drop in platelet count, typically occurring 5 to 14 days after initiation of heparin therapy (or sooner in patients with prior heparin exposure). Platelet counts usually fall by more than 50% from baseline, though they rarely drop below 20,000/μL. Despite the low platelet count, the major clinical risk is not bleeding but rather thrombosis. Patients may develop venous thromboembolism (deep vein thrombosis, pulmonary embolism) or arterial thrombosis (stroke, limb ischemia, myocardial infarction), a condition referred to as heparin-induced thrombocytopenia and thrombosis (HITT). Skin necrosis at heparin injection sites and acute systemic reactions following heparin bolus administration may also occur. Management of classic HIT requires immediate discontinuation of all heparin products and initiation of an alternative non-heparin anticoagulant. Approved therapeutic options include direct thrombin inhibitors such as argatroban and bivalirudin, as well as the factor Xa inhibitor fondaparinux (used off-label in some settings). More recently, direct oral anticoagulants (DOACs) such as rivarubicin have been used in select cases. Warfarin should not be initiated until platelet counts have substantially recovered, as early warfarin use can precipitate venous limb gangrene. Early recognition and appropriate treatment are critical to reducing the risk of life-threatening thrombotic complications.

Data sourced from FDA regulatory filings and ClinicalTrials.gov. Updated periodically.

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