Laboratory Testing for Heparin-Induced Thrombocytopenia and Vaccine-Induced Immune Thrombotic Thrombocytopenia Antibodies: A Narrative Review

Abstract

Heparin-induced thrombocytopenia (HIT) and vaccine-induced immune thrombotic thrombocytopenia (VITT) are highly prothrombotic (thrombosis frequency ≥50%). Both are caused by platelet-activating anti-platelet factor 4 (PF4) antibodies, forming PF4/IgG-containing immune complexes that engage platelet FcγIIa receptors, producing strong platelet activation. In HIT, heparin crosslinks several PF4 molecules, whereas in VITT, anti-PF4 antibodies alone crosslink PF4. Sufficient levels of circulating anti-PF4 antibodies are needed to create the pathogenic immune complexes on platelet surfaces; this explains why certain serum (plasma)-based assays are highly sensitive for detecting HIT/VITT antibodies. Accordingly, HIT and VITT are “clinical-pathological” disorders, that is, positive testing for such antibodies—together with a compatible clinical picture—is integral for diagnosis. Heparin (low concentrations) enhances HIT antibody-induced platelet activation, but platelet activation by VITT sera is usually inhibited by heparin. For both HIT and VITT, high sensitivity (>99% and >95%, respectively) characterizes PF4-dependent enzyme immunoassays (EIAs) and PF4-enhanced platelet activation assays; in contrast, certain rapid immunoassays have high sensitivity for HIT (>90-97%) but poor sensitivity (<25%) for VITT. HIT and VITT antibodies are directed at distinct sites on PF4: solid-phase EIAs and platelet activation assays are indifferent to these distinct antigen targets, but rapid immunoassays are not. We discuss a conceptual model where PF4 is viewed as a “globe,” with the heparin-binding site the “equator”; in this model, HIT antibodies are primarily directed at antigen site(s) at the north and south “poles” of PF4 (formed when PF4 binds to heparin), whereas VITT antibodies recognize sites on the equator.

Platelet factor 4 triggers thrombo‐inflammation by bridging innate and adaptive immunity

“…In summary, three PF4‐enhanced washed platelet assays—PF4‐SRA, PIPA, PEA—appear to have high sensitivity for detecting VITT antibodies (in contrast, the PF4/H‐SRA is preferred for detecting HIT rather than VITT antibodies 62 ). An important caveat is that a false‐negative platelet activation assay can result if the patient was treated with high‐dose IVIG, 16 through inhibition of VITT antibody‐induced platelet activation 71,72 …”

“…The narrow range of heparin is explained by the necessity to create optimal stoichiometric ratios between PF4 and polyanion. Both assays have high diagnostic sensitivity (~95%) and specificity (~95%) for HIT, 62 although much lower sensitivity for VITT (~50%).…”

Evolution of Hemostasis Testing: A Personal Reflection Covering over 40 Years of History*

“…Thus, positivity by AcuStar and by ELISA is more likely HIT/HITT than VITT. Moreover, a separate category of anti-PF4 antibody-associated pathology can also occur due to infection, including SARS-CoV-2, or surgery, 120,[130][131][132] thus further complicating differential diagnosis of VITT versus HITT versus infection/surgery-related thrombotic thrombocytopenia, in patients undergoing surgery, or hospitalized with COVID-19, and/or under heparin treatment.…”

Thromboembolische Erkrankungen aus hämostaseologischer Sicht

“…Wurde zum Zeitpunkt der Thromboseentstehung ein unfraktioniertes oder niedermolekulares Heparin eingesetzt, besteht der Verdacht auf eine heparininduzierte Thrombozytopenie (HIT) [16]. In diesem Fall sollte das Heparin durch eine alternative Antikoagulation ersetzt und eine HIT-Diagnostik eingeleitet werden [17]. Geeignete alternative Antikoagulanzien sind Argatroban und die direkten oralen Antikoagulanzien [18].…”