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- Unexplained decrease in platelet count in a patient treated with heparin
- Minimum platelet count falls between 30% and 50% from baseline.
- Antibody-mediated prothrombotic disorder initiated by heparin administration
- Unlike other thrombocytopenias, heparin-induced thrombocytopenia (HIT) is an idiosyncratic reaction that produces thrombosis rather than bleeding.
- Two types: nonimmune heparin-associated thrombocytopenia (previously called HIT type I) and heparin-induced thrombocytopenia/thrombosis (HITT) (immune induced; previously called HIT type II)
- Nonimmune heparin-associated thrombocytopenia (HIT): more common, onset 1 to 4 days after starting heparin, mild thrombocytopenia (>100,000), few complications
- Immune HITT: less common, onset 5 to 14 days after primary exposure to heparin, thrombocytopenia often <100,000 but usually >20,000; high risk of thrombosis and mortality
- Presentation of thrombocytopenia can be immediate with recent heparin exposure (within past 100 days).
Etiology and Pathophysiology
- Nonimmune heparin-associated thrombocytopenia: potentially a result of direct platelet membrane binding with heparin
- HITT: Heparin can cause an increase in the blood concentration of platelet factor 4 (PF4), a chemokine. PF4 will form a complex with heparin.
- Heparin/PF4 complex can, in turn, stimulate the production of specific antiheparin/PF4 complex antibodies. These antibodies cause platelet activation and a prothrombotic state. Ultimately, this hypercoagulable state leads to thromboembolic complications in many patients.
- Sources of heparin
- Heparin flushes (e.g., for arterial lines or heparin locks)
- Heparin-bonded catheters
- Low-molecular-weight heparin (LMWH) (enoxaparin, dalteparin)
- Postsurgical > medical > obstetric
- Post-cardiopulmonary bypass (CPB) is the most significant risk factor.
- Bovine unfractionated heparin (UFH) > porcine UFH > LMWH
- Female > male
- Heparin duration >5 days
- Inquire about recent heparin exposure and any history of HIT.
- Use of LMWH (vs. unfractionated), for a shorter duration, can reduce the risk of developing HIT.
- Properly document past HIT reactions in patient’s medical record. Develop a HIT recognition and treatment protocol.
- No form of heparin should be administered once the diagnosis of HIT is confirmed.
Commonly Associated Conditions
- Venous thrombosis: deep venous thrombosis (DVT), pulmonary embolism (PE), adrenal vein thrombosis with hemorrhagic infarction; seen more frequently among medical and postoperative orthopedic surgery patients
- Arterial thrombosis: myocardial infarction, stroke, mesenteric infarction, limb ischemia; seen more frequently among vascular and cardiac surgery patients
- Skin lesions (skin necrosis at site of injection)
- Acute systemic reactions