ABSTRACT
Reviewing the literature for studies evaluating the frequency of HIT requires an understanding of the difference between the development of antiplatelet factor 4/heparin (PF4/H) antibodies (immunization) and the development of clinical HIT (platelet-activating anti-PF4/H antibodies that cause thrombocytopenia and/or thrombosis). As illustrated by the Iceberg Model (see Fig. 1.4 in chap. 1 and Fig. 11.9 in chap. 11), only anti-PF4/H antibodies of IgG class that possess platelet-activating properties are associated with risk of clinical HIT. Consequently, there are two different types of HIT frequency studies: (i) studies that evaluated the frequency of immunization by heparin (and related molecules) in specifi c patient populations, and (ii) studies that evaluated the frequency of clinical HIT in patients who are suspected to have this condition based on clinical features (typically, thrombocytopenia).
