HiT II-Ab ELISA is a solid phase enzyme immunoassay for the quantitative and qualitative detection of IgG antibodies that cause heparin induced thrombocytopenia type II. Heparin induced thrombocytopenia (HiT) is a severe side-effect of heparin treatment and occurs in 1-3 % of the treated patients. Two distinct types of heparin induced thrombocytopenia can occur: HiT type I has no clinical relevance and is characterised by a transient decrease in platelet counts. It recovers after a few days even if the administration of heparin continues. HiT type II is an immune-mediated form. The platelet counts drop more than 50% from the baseline 5 to 14 days after first beginning of heparin adminstration. The affected patients develop antibodies that recognize neoepitopes exposed by the complex of platelet factor 4 and heparin. The antibodies found are most commonly of the IgG subclass with or without IgM and IgA class antibodies. IgM and IgA are rarely found without IgG antibodies. However, the pathogenic effect has been proofed for the IgG subclass whereas the effect of IgM and IgA antibodies is still controversially discussed. The Fc portion of the HiT antibody can bind to the platelet Fc receptor and this interaction triggers activation and aggregation of the platelets. Activated platelets release PF4, thus perpetuating the cycle of heparin-induced platelet activation. The platelet activation leads to the production of platelet factor 4 and prothrombotic platelet microparticles. Heparin-like molecules (heparan sulfate) on the surface of endothelial cells are also able to form a complex with platelet factor 4 that can be recognized by HiT antibodies. That, in turn, can induce tissue factor expression with further activation of the coagulation cascade and thrombin generation. It leads to an increased risk for new arterial and venous thromboembolic complications that can be lethal in 10-15 % of the patients. An early diagnosis and a replacement with a suitable alternative anticoagulant can clearly minimize the complication rate.