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Laboratory testing for HIT  

Laboratory tests are useful to confirm or rule out a clinical suspicion of HIT. In case of high clinical likelihood they can also confirm the diagnosis of HIT, but such confirmation should not delay treatment. Laboratory confirmation is usually obtained after treatment has started.

Only a subset of HIT antibodies (PF4-heparin antibodies) have platelet activating properties and only a subset of patients with HIT antibodies will develop the clinical features of HIT. Therefore, patients should not be screened for antibodies unless their clinical profile suggests HIT.

There are two types of assays for HIT (1,2):

  • Antigen assays -  for example PF4 enzyme-linked  immunoassay (ELISA), micro-column agglutination test (e.g.DiaMed gel-card test), platelet adhesion assay (PADA-HIT assay)
  • Functional assays - platelet activation assays using washed platelets e.g. heparin-induced platelet activation assay (HIPAA), serotonin- release assay (SRA)

Antigen assays

Antigen assays such as ELISA are very sensitive for detecting all antibody classes to the PF4-heparin complex, but these assays are much less specific than the functional assays for detecting clinically relevant antibodies and supporting a diagnosis of clinical HIT(1,2).

Functional assays

Platelet activation assays that use washed platelets, such as the heparin-induced platelet activation assay (HIPAA) and the serotonin-release assay (SRA) exclusively detect platelet activating antibodies of IgG, rather than the non-platelet activating IgM and IgA class antibodies(2). These assays have a much greater specificity for detecting clinically relevant HIT antibodies and are better able to predict clinical HIT. Functional assays are technically demanding and are usually restricted to central reference laboratories

1.Selleng K et al. Heparin-induced thrombocytopenia in intensive care patients. Crit Care Med 2007; 35(4):1165-76

2.Warkentin TE, Greinacher A, Koster A, Lincoff AM. Treatment and prevention of heparin-induced thrombocytopenia: ACCP evidence based clinical practice guidelines (8th Edition). Chest 2008; 133:340-380

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Managing the HIT patient