EBM Consult

Lab Test: Heparin Induced Thrombocytopenia Level

    Lab Test
    • Heparin Induced Thrombocytopenia
    • Several available techniques, functional or observational, intended to evaluate possible heparin induced thrombocytopenia (HIT)
    Reference Range
    • Negative as defined by the specific test methodology being utilized
    Indications & Uses
    • Evaluation of suspected (HIT) - the results of type II (HIT) is dependent upon clinical and laboratory evidence.  Diagnostic criteria include evidence of relative or absolute thrombocytopenia (> 50% drop in platelet count relative to pre-heparin levels or a count below 100,000/mm3), the exclusion of other causes of thrombocytopenia (besides heparin administration), and the resolution of thrombocytopenia with discontinuation of heparin therapy.  Diagnostic test interpretation should reflect both the clinical conte4xt and the relative abnormality of HIT antibody testing results.
      • Of key importance is interpretation of the study based on clinical context.  Both types of assays (functional and observational) have high sensitivity and high negative predictive values; however, the specificity varies.  First, not all patients with antibodies have type II HIT; in fact, most patients do not have HIT.  Second, particularly in the observational assay but to some extent in the functional one, there can be cross-reaction with other antibodies, thus making the test nonspecific.  As a result, a positive test needs to be confirmed by the correct clinical setting, including the degree of platelet decline, timing after heparin onset, presence of thrombosis, and presence of other causes of platelet fall.
    Clinical Application
    • The assessment for HIT is done when a patient is receiving heparin or a low-molecular weight heparin and demonstrate a drop in the platelet count < 100,000 or when there has been a >50% reduction from baseline in the patient's platelet count.
    • The drop in platelets during HIT is due to consumption in the formation of a thombus and no from being destroyed.
    • Since antibody assays can be nonspecific, particularly in the postoperative period, interpretation of a positive HIT antibody test should be done in context with the clinical setting.
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.

MESH Terms & Keywords

  • Heparin, Induced, Thrombocytopenia