• Christopher Reilly-Stitt, Steve Kitchen, Ian Jennings, Kevin Horner, Robert Jones, Mike Makris, and Isobel D Walker.
• UK NEQAS (Blood Coagulation), Sheffield, UK.
• J. Thromb. Haemost. 2021 Sep 1; 19 (9): 2263-2267.

BackgroundVaccine-induced immune thrombocytopenia and thrombosis (VITT) following the administration of the AstraZeneca (AZ) ChAdOx1 nCOV-19 vaccine has recently been reported. The associated clinical and laboratory features have included thrombosis at unusual sites, thrombocytopenia, and raised D-dimers with positivity for IgG anti-platelet factor 4 (PF4) antibodies.ObjectivesA UK National External Quality Control Assessment Scheme external quality control exercise was carried out by distributing liquid and lyophilized samples from a subject with VITT, a pool of samples from subjects with classical heparin-induced thrombocytopenia (HIT), and a non-VITT/non-HIT case to 85 centers performing HIT testing.MethodsParticipating centers employed their locally validated testing methods for HIT assays.ResultsThe lyophilized and liquid samples were found to be commutable for the ELISA assays used in the detection of anti-PF4 antibodies. The Aeskulisa, Stago, Hyphen, and LIFECODES anti-PF4 ELISA assays successfully detected the VITT antibody, whereas the Acustar HIT, Werfen LIA, and the Stago STIC assays did not.ConclusionIt is important that clinical and laboratory teams are aware of the limitations of some anti-PF4 assays when using them to aid diagnosis of VITT syndrome.© 2021 International Society on Thrombosis and Haemostasis.

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