Falsely positive heparin-induced thrombocytopenia antibody testing in severe hyperbilirubinemia.

Heparin-induced thrombocytopenia (HIT) is a life-threatening pathologic reaction to heparin-based products. Diagnosis of this condition can be confounded by other comorbidities or by acute illness-oftentimes presenting challenging clinical dilemmas, particularly in critically ill patients. A 67-year-old woman was admitted with liver failure and severe hyperbilirubinemia. She developed thrombocytopenia after prophylactic heparin exposure. Subsequent quantitative latex immunoturbidimetric assay (LIA) HIT antibody testing was intermediately positive. Confirmatory serotonin release assay testing subsequently returned negative. Platelet factor4-dependent P-selectin expression assay also returned negative, suggesting false positivity of the initial LIA tests. Concern was raised that hyperbilirubinemia (total bilirubin, 55.5 mg/dL) interfered with the original assay. Further testing with a separate HIT ELISA assay, which includes multiple washes and dilutions of the serum in order to effectively remove bilirubin, returned negative. Medical providers must consider the possibility of false-positive LIA testing when evaluating for HIT in the setting of severe hyperbilirubinemia.