IN FOCUS: Activation of platelets by heparin-induced thrombocytopenia antibodies in the serotonin release assay is not dependent on the presence of heparin

Abstract

Summary. The serotonin release assay (SRA) tests for antibodies responsible for heparin-induced thrombocytopenia (HIT). By definition, SRA-positive antibodies cause platelet serotonin release in vitro, in the presence of low concentrations of heparin, but not with excess heparin. Many SRA-positive sera activate platelets in the presence of saline without drug, either as a result of residual heparin in the specimen, or because of intrinsic features of the HIT antibodies. The present experiments show that neither exhaustive heparinase treatment, nor chromatographic removal of heparin abrogates the spontaneous platelet activation caused by these HIT antibodies. This is the first study to systematically demonstrate that in vitro activity of HIT antibodies can be independent of heparin. In addition, T-gel chromatography demonstrated differences among fractions of enzyme-linked-immunosorbent assay (ELISA)-positive HIT antibodies within individual specimens. Certain ELISA-positive fractions had SRA activity while others did not, and the SRA activity was not proportional to HIT antibody ELISA titer. These data suggest that antibodies formed as a result of heparin treatment are heterogeneous, and that some can contribute to the pathogenesis of HIT even when heparin is no longer present.

The role of platelets in the pathogenesis of cerebral malaria

“…An example of this is heparin-induced thrombocytopenia where antibodies are formed to heparinplatelet factor 4 complex on the surface of platelets [33]. In some cases, these antibodies lead to platelet activation [34]. Platelet turnover has been shown to be increased in plasmodium-infected patients with thrombocytopenia and platelet life-span reduced [35].…”

Heparin-Induced Thrombocytopenia: A Serious Complication of Heparin Therapy for Acute Stroke

“…The result was considered as positive when the titer was greater than the cutoff value, which was determined using the reference control given for each kit. SRA was done to confirm the diagnosis of HIT retrospectively and was performed as described elsewhere at the Loyola University Medical Center (Maywood, Ill., USA) [20]. For each sample, SRA was done using washed platelets from at least 2 donors, known to be reactive in the assay.…”

Heparin-Induced Thrombocytopenia: Frequency and Pathogenesis

“…Typically, these patients have antibodies which strongly react in vitro with platelets even in the absence of heparin, and that exhibit very high optical densities in the immunoassay. A recent in vitro study showed that at least some antibodies induced by heparin treatment can bind to platelet bound PF4 [45] in the absence of heparin. Using the monoclonal antibody KKO Raouva et al suggest that this is dependent on the density of PF4 on the platelet surface.…”