New concepts in heparin-induced thrombocytopenia: diagnosis and management.

Heparin-induced thrombocytopenia (HIT) is a clinicopathologic condition and adverse drug reaction caused by immunoglobulin G (IgG) antibodies directed against the heparin-platelet factor 4 complex. In most patients, the onset of thrombocytopenia begins while the patient is receiving heparin. In less than 5% of patients, the onset of thrombocytopenia begins several days following heparin discontinuation and has been termed "delayed-onset" HIT. This review summarizes the presentation and clinical course of published reports of delayed-onset HIT occurring in 30 patients. The diagnosis of delayed-onset HIT should be considered in all patients presenting with venous thromboembolism (VTE) and all patients with recent heparin exposure (within the past 14 days) who present with a low platelet count. Most patients with HIT are treated with direct thrombin inhibitors and transitioned to warfarin oral anticoagulation. Administration of direct thrombin inhibitors requires close monitoring for bleeding, dose adjustments based upon coagulation monitoring and is costly. Fondaparinux, a synthetic pentasaccharide and indirect-acting factor-Xa inhibitor, has little to no cross-reactivity with the heparin-platelet factor 4 antibody in in vitro testing. This review summarizes dosing, monitoring and outcomes of preliminary reports of fondaparinux successfully administered to 13 patients with subacute HIT and 22 patients with acute HIT. While several reports have described the treatment and prophylaxis of thrombosis in patients with HIT using fondaparinux, clinical trials should be conducted and reported before fondaparinux becomes a therapy of choice for HIT.