Audit of pathology requesting behaviour in the setting of vaccine-induced immune thrombotic thrombocytopenia (VITT)

Abstract

Background: Vaccine-induced immune thrombotic thrombocytopenia (VITT) following AstraZeneca SARS-CoV2 vaccine was first reported in February 2021.1 Algorithms using D-dimer and thrombocytopenia informed further testing by ELISA for PF4 antibodies.2,3 Community awareness of VITT was predicted to influence patterns of requesting. Method: Audit of the influence of VITT on D-dimer requests at a large private pathology provider. Results: A sharp increase in D-dimer requests was observed between 05/21 and 06/21, influencing total test numbers for the year: 2019 (6256), 2020 (6215), 2021 (10008). June 2021 had a higher proportion of results within normal range (<0.5 μg/mL), and a higher proportion of tests in the outpatient setting compared to 2019/2020. VITT was the clinical indication in 37% of episodes in June 2021. 41 cases were referred to our reference centre for PF4 ELISA testing in 2021; 8 of these patients were positive by PF4 ELISA. Conclusion: This audit demonstrates that concern for VITT stimulated an increase in D-dimer testing. However, a very low proportion of patients fulfilled clinical or laboratory criteria for confirmatory testing. 20% of patients preceding to confirmatory testing returned positive results, suggesting high specificity of the diagnostic algorithm and triage process.