How low can a platelet count get before you suspect heparin-induced thrombocytopenia (HIT)?
—Higinio R. Lopez, MD, Newark, Del.

During heparin therapy, any unexplained decrease that reduces the platelet count 50% or to <150,000/µL should trigger an antiheparin/platelet factor (PF) 4 antibody determination. The mild form of disease, also known as “benign” or “type II thrombocytopenia” (platelet count 100,000-150,000/µL), has a rapid onset (one to two days), resolves despite continuation of heparin, is typically asymptomatic, and is thought to reflect a nonimmune mechanism, since tests for the antiheparin/PF4 antibody are negative. Type I heparin-induced thrombocytopenia (platelets usually <100,000/µL) often has delayed onset (4-14 days), persists until heparin is discontinued, carries a 50% risk for thromboembolic complications, and is antiheparin/PF4 antibody-positive. A single platelet count may not predict the type. For additional information, see Lichtman MA, Beutler E, Seligsohn U, et al, eds. Williams Hematology. 7th ed., New York, N.Y.: McGraw-Hill; 2005:2041, 2062.
—Dennis K. Galanakis, MD (104-8)

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