Confirmatory tests

Several reliable assays are available to diagnose HIT. The basis of the platelet C-serotonin-release assay (SRA) is that antibodies from patients with HIT will cause platelet activation and release of C-serotonin from platelet-dense granules when HIT serum is incubated with normal donor platelets at a therapeutic concentration of heparin²⁰. The test can be made more specific for HIT by showing that platelet release is inhibited by high concentrations of heparin, which cause displacement of immune complexes from the platelet surface, or by pre-incubation with a platelet Fc receptor-blocking monoclonal antibody²⁰.

Sensitivity and specificity of this assay for HIT are greater than 90%.

Patients who formed heparin-dependent antibodies detected by the SRA were much more likely (odds ratio 78.2, 95% CI 12.0-818.8, p<0.001) to develop thrombocytopenia than controls.

Disadvantages of this test are use of radioactive substances and its technical demands.

A related assay is the heparin-induced platelet-activation assay (HIPAA)²¹. Like the SRA, it employs washed platelets from normal donors. However, rather than having serotonin release as the diagnostic end point, it assesses aggregation of platelets induced by patient serum in the presence of heparin. The assay is rapid and approximately as sensitive as SRA.

Both tests can determine whether a given patient's heparin-dependent antibodies cross-react with other anti-coagulant polysaccharides, such as LMWH^21,22. Platelet-rich plasma (PRP) aggregation is simple, rapid, and widely performed to test for HIT. Unfortunately, its sensitivity may be as low as 50%^16,21.

The discovery that the major antigen of HIT was the heparin-PF4 complex led to development of an enzyme-linked immunosorbent assay (ELISA) to detect HIT-specific antibodies^8,16. This assay can detect IgG, IgM, and IgA antibodies, and has sensitivity of approximately 80-90%^16,23. Sensitivity is less than 100% because the test does not recognize antigens other than heparin-PF4 complex. Furthermore, the assay cannot be used to investigate cross-reactivity of HIT sera for other anticoagulant polysaccharides.

Generic tests for platelet-associated IgG are not helpful in diagnosing HIT because of their low specificity (i.e., platelet-associated IgG is elevated in most thrombocytopenic disorders)⁴.

© 2008 Organon Laboratories. All rights reserved. Date of prep. November 2008. Item code: 09847G