Duration of Therapy

Because of the persistent high (38% to 76%) risk of thrombosis for at least a month after onset of HIT (Hirsh et al., 2004), nonheparin anticoagulation should be maintained for at least 4 wk (Davoren and Aster, 2006; Arepally and Ortel, 2006). A longer duration, e.g., 3-6 mo, should be considered after an episode of HIT-associated thrombosis. According to treatment guidelines, anticoagulation with an alternative parenteral agent such as argatroban should be continued at least until platelet count recovery (a count of at least 100 x 109/L, preferably 150 x 109/L) and further overlapped with warfarin for a minimum of 5 days and until warfarin effects have been therapeutic for at least 2 days (Warkentin and Greinacher, 2004). These guidelines are important to ensure continuous anticoagulation and to avoid prothrombotic effects of initiating warfarin during acute HIT, e.g., warfarin-induced venous limb gangrene or skin necrosis syndromes (see Chapter 2). In argatroban-treated patients with HIT in study Argatroban-915, mean platelet counts were >100 X 10 /L after 2 days of therapy and >150 X 109/L after 4 days of therapy (Lewis et al., 2003).

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