Negative heparin-induced thrombocytopenia test result after massive transfusion: Believe it or not?

Objectives: Diagnosis of heparin-induced thrombocytopenia (HIT) is complicated by a high false-positive rate for the screening enzyme immunoassay (EIA) and limited availability of confirmatory platelet activation assays such as serotonin release assay (SRA). We evaluate the impact of a massive transfusion on EIA and SRA testing and emphasize that the timing of the confirmatory sample is important. Methods: We present a case in which separate samples for HIT testing were collected before and after a major bleed requiring massive transfusion. We also discuss a recent study in which HIT serum samples were diluted in vitro and in vivo. Results: The EIA was strongly positive, but SRA was negative, leading to suspicion of a false-positive EIA result. However, SRA performed on the initial EIA specimen was strongly positive. A second EIA, drawn after a massive transfusion, was negative. Conclusions: Replacement of several blood volumes diluted the HIT antibodies below the limit of detection. Confirmatory testing for HIT antibodies should be done on the specimen that initially tested positive.