CONCLUSIONS AND RECOMMENDATIONS
The patient described in the vignette had a marked decrease in the platelet count after several days of therapy with low-molecular-weight heparin, which raises concern about HIT. Calculation of the 4T score is recommended to determine her risk of HIT. Her score of 5 points (decrease in platelet count, 2; timing, 2; thrombosis, 0; and likelihood of other reasons, 1, since her endocarditis is stable and the platelet count is too high for antibiotic-induced immune thrombocytopenia) places her at intermediate risk. Although routine screening for PF4–heparin antibodies is strongly discouraged, patients at intermediate or high risk should undergo this testing. A positive anti–PF4–heparin IgG enzyme immunoassay is necessary for the diagnosis of HIT but is nonspecific. A strongly positive test (optical density, >1.5) or positive platelet-activation assay would strongly support the diagnosis of HIT. Treatment involves the prompt cessation of heparin and the initiation of an alternative anticoagulant (argatroban or danaparoid, both of which are approved for this indication, or fondaparinux or bivalirudin, with use of these agents supported by case series).