Laboratory testing for VITT antibodies

Semin Hematol. 2022 Apr;59(2):80-88. doi: 10.1053/j.seminhematol.2022.03.003. Epub 2022 Mar 7.

Abstract

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a highly prothrombotic disorder that like heparin-induced thrombocytopenia (HIT) is caused by platelet-activating antibodies that recognize platelet factor 4 (PF4). However, unlike HIT-where heparin at low concentrations (0.1-0.5 U/mL) typically enhances antibody-induced platelet activation, platelet activation by VITT sera is usually inhibited by heparin. Further, conventional platelet activation assays for HIT, such as the serotonin-release assay (SRA) and heparin-induced platelet activation (HIPA) test, often yield negative or atypical results when testing VITT sera. Nevertheless, VITT (like HIT) is a "clinical-pathological" disorder whereby laboratory detectability of platelet-activating anti-PF4 antibodies is crucial for diagnosis. VITT antibodies follow 2 fundamental principles of HIT laboratory testing: (1) high probability of a positive PF4-dependent enzyme-immunoassay (EIA), and (2) high probability of a positive platelet activation assay. However, optimal detection of VITT in platelet activation assays requires the addition of PF4, for example, PF4-enhanced SRA (PF4-SRA) and PF4-enhanced HIPA (PIPA). A novel whole blood assay, called the PF4-induced flow cytometry-based platelet activation (PIFPA) assay, exhibits high sensitivity and specificity for VITT. HIT and VITT sera/plasmas differ in their reactivity in rapid HIT immunoassays (90-97% sensitivity for HIT, <25% sensitivity for VITT), consistent with distinct antigen sites on PF4 recognized by HIT and VITT antibodies.

Keywords: Enzyme-immunoassay (EIA); PF4-induced flow cytometry-based platelet activation (PIFPA) assay; Platelet factor 4 (PF4); Platelet-activating antibodies; Vaccine-induced immune thrombotic thrombocytopenia (VITT).

MeSH terms

  • Antibodies* / analysis
  • Heparin / adverse effects
  • Humans
  • Platelet Factor 4
  • Purpura, Thrombocytopenic, Idiopathic* / chemically induced
  • Vaccines* / adverse effects

Substances

  • Antibodies
  • Vaccines
  • Platelet Factor 4
  • Heparin