This Cannot Be HIT Because It Is Too Early Too Late or the Platelets Are Not Low Enough or They Are Too

"Classic" HIT ensues 5-10 days (occasionally, a few days later) after the beginning of a course of heparin, but rapid-onset HIT occurs sooner, sometimes within minutes of heparin administration, if the patient has had prior sensitization to heparin within the preceding 3 mo (Warkentin and Kelton, 2001a; Mims et al., 2004). While the degree of thrombocytopenia with HIT is often moderate (median platelet count, 60 X 109/L), 10% of patients may experience severe thrombocytopenia (platelet count less than 20 X 109/L) (Warkentin, 2003, 2007); remarkably such patients rarely bleed, even though many are fully anticoagulated, and in fact those patients with the lowest platelet counts have the highest risk for thromboembolic complications and thus the most dire need for alternative anticoagulation. Another 10% of HIT patients may have nadir platelet counts that fall within the normal range, but usually these are patients that had substantially elevated platelet counts a few days earlier (Warkentin et al., 2003). Thus, one should not completely dismiss the possibility of HIT when temporal features or the magnitudes of the platelet count decline are atypical.

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