Heparin‐induced thrombocytopenia (HIT) is a potentially catastrophic complication
of heparin therapy. The syndrome is the result of the production of an antibody to
the complex that forms between heparin and platelet factor 4 (H‐PF4) released from
activated platelets. IgG antibodies bind to platelet Fc receptors and cause platelet
activation, resulting in thrombocytopenia and greatly increased risk of thrombosis.
Tests for the H‐PF4 antibody can be classified into functional assays (which rely
on the demonstration of platelet activation) and immunoassays (which detect the presence
of an antibody without regard for its functional ability). The functional assays have
a greater specificity for clinical HIT, but require normal donor platelets and are
relatively unstandardized. The immunoassays have the advantage of better standardization
and do not require normal platelets, but may give positive results in the absence
of clinical HIT. The choice of test is usually dictated by what is possible for a
given laboratory in terms of instrumentation, expertise, and interest. For most institutions
this will be a commercially available enzyme‐linked immunosorbent assay. Although
HIT is a true clinicopathological syndrome, there are several reasons why its diagnosis
still rests primarily on clinical grounds. First, laboratory tests may not be available
locally. Second, they may not be available in a sufficiently timely manner. Finally,
available tests are not completely sensitive or specific for the condition. There
is, therefore, a continuing need to develop more rapid testing strategies with greater
specificity for clinical HIT.
KEYWORDS
Heparin‐induced thrombocytopenia - HIT - platelet counts - heparin‐platelet factor
4 antibodies - laboratory diagnosis
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Dr.
John L FrancisPh.D.
Florida Hospital Center for Hemostasis and Thrombosis
2501 North Orange Avenue, Suite 786, Orlando, FL 32804
eMail: john.francis@flhosp.org
