ABSTRACT
Introduction Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare, but severe side effect after Covid-19 and other vaccinations. First cases of VITT-mimicking antibodies in unvaccinated patients with recurrent thrombosis have been described. Differentiation between heparin-induced thrombocytopenia (HIT) and VITT is difficult in some patients. Widely used enzyme-linked immunoassays (EIA) cannot differentiate between the two, some of them even fail to detect VITT antibodies. So far, differentiation between HIT-like and VITT-like anti-PF4 antibodies can only be performed in specialized laboratories by functional tests using the heparin-induced platelet activation (HIPA) or PF4-induced platelet activation (PIPA) test. We have developed an assay, which can distinguish between HIT and VITT antibodies and can be used in any hospital laboratory. Method Confirming platelet-activation assays (HIPA and PIPA) were performed as described.[1] We defined 3 cohorts: 1) Negative controls (n = 112, including 35 healthy donors from before 2020, 46 clinical patients suspected for HIT but with negative EIA and HIPA and 31 non-thrombotic patients);2) classical HIT-patients with positive EIA and HIPA (n = 121);3) typical VITT patients (n = 63;presenting after vaccination with adenoviral vector-based Covid-19 vaccine and positive EIA and PIPA). Samples were analyzed by an automated coagulation analyzer ACL AcuStar (Werfen / IL Inc., Bedford, MA, USA) using HemosIL AcuStar HIT-IgG(PF4-H) and a prototype of VITT-IgG(PF4) assay according to the manufacturer's protocol. For both assays, raw data was analyzed as relative light units (RLU). Results All VITT samples were positive in the prototype VITT-assay (Fig. 1);only a few (n = 9;14.3 %) also showed weakly positive results in the HIT-assay. On the other hand, most of the HIT samples showed positive results in the HIT-assay (113;93.4 %), 34 of them (30.1 %) also reacted positive in the prototype VITT-assay (12 of them strongly;10.6 %), and three demonstrated an antibody pattern like autoimmune VITT. Negative control samples where all non-reactive in the HITassay and served to adjust the cutoff for the prototype VITT-assay. Conclusion The different reaction pattern of samples of HIT and VITT patients using HemosIL AcuStar HIT-IgG(PF4-H) and a VITT prototype assay was able to distinguish between the two antibody entities for the first time. The combination of assays can facilitate a rapid decision whether heparin may be used for treatment and also identify patients with autoimmune-VITT as a cause of recurrent thrombosis. (Table Presented).