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Chapter 109. Disorders of Platelets and Vessel Wall (Part 5)

Laboratory Testing for HIT HIT (antiheparin/PF4) antibodies can be detected using two types of assays. The most widely available is an enzyme-linked immunoassay (ELISA) with PF4/polyanion complex as the antigen. Since many patients develop antibodies but do not develop clinical HIT, the test has a low specificity for the diagnosis of HIT. This is especially true in patients who have undergone cardiopulmonary bypass surgery, where approximately 50% of patients develop these antibodies postoperatively. The other assay is a platelet activation assay that measures the ability of the patients' serum to activate platelets in the presence of heparin in a. | Chapter 109. Disorders of Platelets and Vessel Wall Part 5 Laboratory Testing for HIT HIT antiheparin PF4 antibodies can be detected using two types of assays. The most widely available is an enzyme-linked immunoassay ELISA with PF4 polyanion complex as the antigen. Since many patients develop antibodies but do not develop clinical HIT the test has a low specificity for the diagnosis of HIT. This is especially true in patients who have undergone cardiopulmonary bypass surgery where approximately 50 of patients develop these antibodies postoperatively. The other assay is a platelet activation assay that measures the ability of the patients serum to activate platelets in the presence of heparin in a concentration-dependent manner. This test has lower sensitivity but higher specificity than the ELISA. However HIT remains a clinical diagnosis. The main value in testing is in excluding the diagnosis with negative tests particularly ELISA. Heparin-Induced Thrombocytopenia Treatment Early recognition is key in treatment of HIT with prompt discontinuation of heparin and use of alternative anticoagulants. Thrombosis is a common complication of HIT even after heparin discontinuation and can occur in both the venous and arterial systems. In patients diagnosed with HIT imaging studies to evaluate the presence of thrombosis at least lower-extremity duplex dopplers are recommended. Patients requiring anticoagulation should be switched from heparin to an alternative anticoagulant. The direct thrombin inhibitors DTIs argatroban and lepirudin are effective in HITT. The DTI bivalirudin and the antithrombinbinding pentasaccharide fondaparinux appear to be effective but are not yet approved by the U.S. Food and Drug Administration FDA for this indication. Danaparoid a mixture of glycosoaminoglycans with anti-Xa activity has been used extensively for the treatment of HITT it is no longer available in the United States but is in other countries. HIT antibodies cross-react with LMWH and