COVID-19 (
coronavirus disease 2019) represents a pandemic, and several
vaccines have been produced to prevent
infection and/or severe sequelae associated with SARS-CoV-2 (
severe acute respiratory syndrome coronavirus 2) infection. There have been several reports of infrequent post
vaccine associated thrombotic events, in particular for adenovirus-based
vaccines. These have variously been termed VIPIT (
vaccine-induced prothrombotic
immune thrombocytopenia), VITT (
vaccine-induced [immune] thrombotic
thrombocytopenia), VATT (
vaccine-associated [immune] thrombotic
thrombocytopenia), and TTS (
thrombosis with
thrombocytopenia syndrome). In this report, the laboratory test processes, as utilised to assess suspected VITT, are reviewed. In published reports to date, there are notable similarities and divergences in testing approaches, potentially leading to identification of slightly disparate patient cohorts. The key to appropriate identification/exclusion of VITT, and potential differentiation from
heparin-induced
thrombocytopenia with
thrombosis (HITT), is identification of potentially differential test patterns. In summary, testing typically comprises platelet counts,
D-dimer,
fibrinogen, and various immunological and functional assays for
platelet factor 4 (PF4)
antibodies. In suspected VITT, there is a generally highly elevated level of
D-dimer,
thrombocytopenia, and PF4
antibodies can be identified by ELISA-based assays, but not by other immunological assays typically positive in HITT. In addition, in some functional platelet activation assays, standard doses of
heparin have been identified to inhibit activation in suspected VITT, but they tend to augment activation in HITT. Conversely, it is also important to not over-diagnose VITT, given that not all cases of
thrombosis post vaccination will have an immune basis and not all PF4-ELISA positive patients will be VITT.