Clinical Care Pathway for the Evaluation of Patients with Suspected Vaccine-Induced Immune Thrombotic Thrombocytopenia After ChAdOx1 n-CoV-19 vaccination

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare complication after SARS-CoV-2 adenoviral vector vaccination. In British Columbia, a provincial clinical care pathway was developed to guide clinicians in evaluating for VITT among patients who present with thrombocytopenia or thrombosis symptoms within 4 to 28 days after adenoviral vector vaccine exposure. All patients had enzyme-linked immunosorbent assay (ELISA) testing for platelet factor 4 (PF4) antibodies and all cases with positive PF4-ELISA or D-dimer levels ≥2.0 mg/L fibrinogen equivalent units (FEU) had further testing for platelet-activating PF4 antibodies using a modified serotonin release assay (SRA). Between May 1 and June 30, 2021, 37% of 68 patients investigated for VITT had thrombosis but only 3 had VITT confirmed by PF4-ELISA and SRA. Platelet counts, D-dimer levels, and ELISA optical density values were significantly different between those with and without VITT. Three patients had thrombocytopenia and thrombosis with D-dimer levels >4.0 mg/L FEU but had negative PF4-ELISA and SRA results. Patients with VITT were treated successfully with intravenous immunoglobulin, non-heparin anticoagulants and corticosteroids. Our pathway demonstrated that thrombosis is common among patients investigated for VITT and that PF4-ELISA testing is necessary to confirm VITT in those presenting with thrombosis and thrombocytopenia.