An update on evidence based diagnostic and confirmatory testing strategies for heparin induced thrombocytopenia using...

This manuscript aims to provide a concise review on current diagnostic/ confirmatory strategies of Heparin Induced Thrombocytopenia (HIT) with the combined use of immunological / functional assays in addition to the clinical probability. Laboratory diagnosis of HIT is of primordial importance as the related complications could become rapidly severe and life-threatening and can provoke limb amputation in some cases. The first action in the presence of HIT suspicion is to withdraw heparin and to initiate an alternative anticoagulant. Whilst vitamin K antagonists are not appropriate, anticoagulant options include Fondaparinux, Sodium Danaparoid, DOACs, Argatroban, and Bivalirudin. However, if HIT is excluded, patients can benefit again from the high therapeutic and antithrombotic efficacy of this drug, which remains superior to all the substitutive anticoagulant treatments. HIT is suspected in the presence of a platelet count drop > 50% on 2 successive counts, or a platelet count < 100 G/L, and of a significant clinical probability (4 Ts score). Testing patients' plasma is required for establishing the diagnosis. Laboratory investigation involves first the immunological measurement of heparin dependent IgG antibodies (mainly targeted to Heparin-Platelet Factor 4 complexes). When positive, a functional assay for platelet activation, performed at a low and high heparin concentration, allows confirming this disease. In any case, if the immuno-assay is negative, HIT can be excluded with a high probability, and heparin can be continued (if clinical examination favors this decision). Conversely, the higher the IgG antibody concentration is (and affinity), the higher is the probability of developing HIT. The functional assay has now become for confirming the platelet activation capacity of antibodies, and therefore confirming the presence of HIT. Up to now, the gold reference method for testing antibody-dependent platelet activation is the C14-Serotonin Release Assay, available only in very few laboratories working with radio-isotopes. A simple, sensitive, and accurate flow cytometry assay becomes now available to all clinical sites, and it can be easily used for testing the capacity of heparin dependent-antibodies to activate platelets, at low heparin concentration. This technique can be performed in any laboratory equipped with a flow cytometer and can make the HIT confirmation diagnosis rapidly available, which introduces a great improvement for management of patients with HIT. We believe that an evidence-based update on this topic is timely and well warranted.