Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (982 page)

   Limitations

   Great attention must be paid in preparing the plasma to be tested in order to eliminate platelet contamination (platelets when activated release platelet factor 4, a potent antiheparin protein). Careful and adequate centrifugation is necessary.

   Spuriously low results may be seen in patients with antithrombin deficiency.

HEPARIN-INDUCED THROMBOCYTOPENIA (HIT) ASSAYS
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   Definition

   HIT refers to thrombocytopenia that develops during or following the administration of heparin. There are various assays in use, none entirely satisfactory.

   There are two groups of assays:

   Immunologic: ELISA assays: use specific IgG antibodies; these assays have a high negative and positive predictive value.

   Functional: Serotonin release assay, the gold standard for diagnosing HIT. An alternative functional assay is platelet aggregation standardized to use heparin as the aggregating agent.

   
Normal values

   ELISA: negative if <0.4 optical density

   Serotonin release assay (depends on the laboratory’s own methodology): negative or positive

   Use

   An HIT assay should be performed whenever HIT is clinically suspected.