BACKGROUND: Vaccine-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. OBJECTIVES: A 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. METHODS: Participating centers employed their locally validated testing methods for HIT assays. RESULTS: The 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. CONCLUSION: It 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.