Anti-Platelet4 Antibodes Testing in Pandemic: Which Test to Use

ABSTRACT

Introduction: Heparin-induced thrombocytopenia (HIT) is a severe adverse reaction to heparin caused by heparin-dependent, platelet-activating anti-plateletfactor 4 (PF4)/heparin antibodies. Vaccine-induced thrombocytopenia and thrombosis (VITT) following Astrazeneca vaccine has been described, associated with IgG anti-PF4 antibodies. Only ELISA immunoassays have been shown to detect anti-PF4 antibodies in these patients. Diagnosis of both HIT and VITT requires confirmation of heparin-dependent, platelets activating antibodies to avoid overdiagnosis and overtreatment Anti-PF4 laboratory assay requests during pandemic are in most of the cases of COVID positive vaccinated subjects, vaccinated healed from COVID19 subjects and COVID negative vaccinated subjects. Aim of our study was to investigate which laboratory test to use in patients with suspected HIT in this particular historical period Methods: Thirty patients with suspected HIT/ VITT were tested with three anti-PF4 immunoassays (HIT Ab Latex Immunoassay-Werfen, Acustar HITIgG CliA-Werfen, HPIA ELISA-Stago) 8 Astrazenca suspected VITT;1 Moderna and 1 Pfizer suspected VITT, 20 suspected HIT (9/20 COVID positive and 11/20 COVID negative patients). In order to confirm immunoassay positivity Platelet Aggregation Test (PAT) functional test was performed in all patients found to be positive for at least one assay. Result(s) 3/8 suspected Astrazenca VITT tested positive only by ELISA assay 2/3 tested positive by PAT (confirmed VITT). Both patients with suspected Pfizer and Moderna VITT tested negative for all immunoassays. 4/9 COVID patients with suspected HIT tested positive only by ELISA assay, only 1 of them tested positive by PAT. 4/11 COVID negative suspected HIT tested positive by all immunoassays performed and 1/11 tested positive only by ELISA and CliA immunoassays, among these 5 subjects only 2 were confirmed HIT by PAT functional assay Conclusion(s) With the only exception of suspected Astrazeneca VITT no superiority of IgG-ELISA over CliA or Latex immunoassay in sensitivity to HIT was observed. No single immunoassay method detected all probable HIT cases;if a single test is negative, a second immunoassay or a platelet activation assay should be considered where there is strong clinical suspicion Interestingly by using Bayesan diagnosis of HIT and CliA conservative negative cut-off < 0.13 U/ml suggested in the literature (Marchetti et al. 2020) all our CliA negative (results < manufacturer's cut-off of 1.0 U/mL) and ELISA positive suspected HIT/VITT were correctly classified as positive by CliA assay.