Multi site comparative study of three heparin induced thrombocytopenia (HIT) test methods and preliminary consideration of their utility in suspected vaccine induced thrombocytopenia and thrombosis (VITT)

ABSTRACT

Heparin induced thrombocytopenia (HIT) occurs when patients generate an antibody response against the heparin/platelet factor 4 (PF4) complex. In some cases, these antibodies cause platelet activation and clinical thrombosis. The 'Four-T's' score assesses clinical likelihood of HIT. Anti-PF4 antibodies have been detected in patients suffering from vaccine-induced immune thrombotic thrombocytopenia (VITT) following SARS-CoV-2 vaccination and testing by ELISA is recommended. Various platelet activation and pF4/heparin complex antigen assays are employed to detect HIT antibodies. High test sensitivity increases likelihood of detecting of antibodies that do not cause clinical HIT. We compared the BioRad ID-PGIA test with the Werfen Acustar assay and the Werfen immunoassay in a comparative study of 12? HIT patients and two UKNEQAS supplementary exercises. The specificity of the BioRad ID-PGIA Heparin/PF4 gel filtration test is considered poor, although its negative predictive value is more reliable. The Werfen Acustar chemiluminescence assay and the Werfen HaemosIL HIT-ab latex immunoassay have comparable sensitivity and superior specificity to the BioRad test. Preliminary studies suggest all three assays have limited sensitivity to VITT. BioRad HIT results were positive for 10/12 patient samples. The two negative samples were also negative by Acustar and immunoassay. One additional sample was negative by Acustar but positive by immunoassay. Four further samples were negative by immunoassay but positive by Acustar. Patients deemed HIT positive by Acustar were switched to fondaparinux or argatroban. Continued heparin infusion did not cause clinical deterioration in the patient who tested HIT positive by immunoassay but not Acustar. The BioRad method obtained expected results for UKNEQAS distributions S20 and S21. Both Werfen methods obtained expected results for S20 but demonstrated poor sensitivity to VITT and HIT in S21. Agreement between results of the three HIT assays is limited but typical of expected patterns. Heparin is variable in terms of chain length and degree of sulphation, which affects degree of conformational change, and therefore antigenic presentation, of PF4 in complex with heparin. Variable antigenic profile in different assays leads to poor comparability of results. It is important to distinguish between HIT and VITT for the purposes of assay selection.