Comparison of 2 functional assays (HIMEA and HIPA) for the diagnosis of vaccine-induced immune thrombocytopenia and thrombosis (VITT): A franco-greek collaborative study

ABSTRACT

Background: Vaccine-induced immune thrombosis and thrombocytopenia (VITT) has been recognized as a rare thrombotic complication of adenovirus-based vaccines against COVID-19. While VITT presents some common features with heparin-induced thrombocytopenia (HIT), VITT antibodies differ from typical HIT antibodies by their ability to induce spontaneous platelet aggregation in the absence of heparin. In this context, reliable functional tests are of crucial importance to confirm VITT diagnosis. Aim(s) To compare the performances of Heparin-Induced Multiple Aggregometry (HIMEA) and Heparin-Induced Platelet Activation assay (HIPA) for VITT diagnosis. Method(s) From April to October 2021, 7 patients meeting the 5 following criteriae of definite VITT were included Onset of symptoms 5-42 days after vaccination, thrombosis, thrombocytopenia, D-dimers >4000 ng/mL and positive anti-PF4 IgG (Zymutest HIA IgG and/or Lifecodes PF4 IgG). HIMEA was performed on whole blood from healthy donors in the hematology laboratory of Alexandra General Hospital. HIPA was performed on washed platelets from healthy donors in the hematology laboratory of Bichat Hospital. Result(s) 4 patients showed a characteristic pattern that was concordant between the two methods. HIMEA showed a sigmoid curve with elevated area under the curve (mean AUC 189+/-78 AU) in saline, that was lower (mean 148+/-51 AU) in the presence of heparin 1 IU/mL and further decreased (mean 34+/-28 AU) in the presence of heparin 100 IU/mL. HIPA showed positive aggregation in the presence of saline and heparin 0,2 IU/mL but no aggregation with heparin 50 IU/mL. 2 patients who showed lower AUC in HIMEA saline (109 and 60 AU) had weakly positive or negative results with HIPA. In these 2 cases, blood was drawn after intravenous immunogloblulin (IVIG) initiation. 1 patient showed discrepant results with typical positive HIMEA but negative HIPA patterns. Conclusion(s) VITT antibodies show heterogenous reactivity patterns that can be highlighted only by using different functional assays such as HIMEA and HIPA. IVIG treatment can be reponsible for lower antibody reactivity.