Iceberg Model of HIT

The interrelationship between antibody formation and clinical HIT (with or without thrombosis) can be illustrated using the "iceberg model" (see Fig. 3). In this model, only anti-PF4/heparin antibodies of IgG class that possess platelet-activating properties are associated with risk of HIT. Analysis of RCT data illustrates that the difference in risk of HIT between UFH and LMWH can be explained by the combination of a lower frequency of antibody formation (whether detected by EIA or SRA), as well as lower risk of "breakthrough" to clinical HIT among patients who have formed heparin-dependent platelet-activating antibodies.

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