HeparinInduced

Heparin-induced thrombocytopenia (HIT), sometimes described as heparin-induced thrombocytopenia and thrombosis (HITT), is an immune-mediated adverse reaction to heparin therapy that can produce a dangerous drop in platelets and a paradoxical risk of thrombosis. There are two main types: a nonimmune, usually mild type I that occurs within the first days of exposure and typically resolves without intervention; and a more serious immune-mediated type II, which occurs after about 5 to 14 days of exposure and can cause arterial and venous clots.

The pathophysiology centers on IgG antibodies directed against complexes of platelet factor 4 (PF4) and heparin. These antibodies form immune complexes that bind to Fc gamma receptor IIa on platelets, triggering robust platelet activation, release of procoagulant microparticles, and thrombin generation. This hypercoagulable state leads to thrombosis despite falling platelet counts. Additional manifestations can include skin lesions at injection sites and, in rare cases, microvascular complications.

Clinical features typically include a drop in platelet count by more than 50% from baseline and new thrombotic events such as deep vein thrombosis or pulmonary embolism. Onset is usually 5–14 days after starting heparin, but can be sooner with recent exposure. Diagnosis combines clinical assessment with laboratory testing: a high-sensitivity 4Ts scoring system guides pretest probability, while laboratories detect anti-PF4/heparin antibodies by ELISA and confirm with functional assays such as the serotonin release assay or heparin-induced platelet activation test.

Management requires stopping all heparin products immediately and initiating alternative anticoagulation with non-heparin agents (for example, argatroban, bivalirudin, fondaparinux, or certain direct oral anticoagulants) depending on the setting. Platelet transfusions are reserved for life-threatening bleeding. Early recognition and treatment improve outcomes; the condition remains a major consideration in patients with unexplained thrombocytopenia during or after heparin exposure.