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Use of the ID-PaGIA Heparin/PF4 Antibody Test as a screening test for heparin/platelet factor 4 antibodies.

Parts of this study were presented as a poster contribution at the 47th Annual Meeting of the German Society of Transfusion Medicine and Immunohaematology [Jahrestagung der Deutschen Gesellschaft für Transfusionsmedizin und Immunhämatologie], September 9th to 12th, 2014, in Dresden, Germany. Introduction Heparin-induced thrombocytopenia type 2 (HIT-2) is an acquired prothrombotic disease, most frequently caused by antibodies against a complex of heparin and platelet factor 4 (PF4) and rarely by antibodies against other chemokines, such as interleukin-8 and neutrophil activating peptide-2, which can also form complexes with heparin1,2. Furthermore, antibodies against protamine can cause a similar adverse drug reaction3. Only antibodies of the immunoglobulin class IgG are relevant for the pathogenesis of HIT-2, because they can bind through their Fab part to macromolecular heparin/PF4 complexes on the surface of the platelets and through their Fc part to the FcγRII-A of the platelets and can thereby activate the platelets1,4. This activation causes consumption of platelets and thrombocytopenia as well as activation of plasma coagulation by release of platelet microparticles which can lead to venous thrombosis and, less frequently, also arterial thrombosis. HIT-2 is induced by unfractionated heparin much more frequently than by low molecular weight heparin5. Most algorithms for the diagnosis of HIT-2 start with an evaluation of the clinical probability based on a scoring system2,6. If the pre-test probability is medium or high, heparin must be replaced and laboratory investigations started6. The next step is different in some algorithms. Sometimes an enzyme immunoassay (EIA) for the detection of antibodies of all immunoglobulin classes (IgG, IgM, IgA) against heparin/PF4 complexes or the particle gel immunoassay (PaGIA), which is also not specific for antibodies of the IgG class, is recommended, with equal ranking, as the first assay2. Sometimes an EIA specific for antibodies of the IgG class against heparin/PF4 complexes is preferred as the first assay6. If an EIA for the detection of IgG/IgM/IgA antibodies is used first and gives a positive result, the next test in the algorithm is an EIA specific for IgG antibodies2. The last step to complete the diagnosis is a functional assay, e.g. the serotonin release assay or the heparin-induced platelet activation assay2,6. Rapid diagnosis of HIT-2 is important, because it is an acute, sometimes life-threatening disease given its possible thromboembolic complications. Most EIA are time consuming and cannot be performed routinely in a standard laboratory every time. However, a preliminary diagnosis of HIT-2 can also be made in non-specialised laboratories by using several simple, less time-consuming, commercially available tests for the detection of heparin/PF4 antibodies, for example the PaGIA, which can be performed in any immunohaematology laboratory using the same type of gel cards as those used for blood group serological examinations7. The aim of our study was, therefore, to evaluate the use of the PaGIA as a step before an EIA specific for antibodies of the IgG class in the diagnostic algorithm for HIT-2.