PB0349 - Anti-PF4-antibodies in patients suspected of vaccine-induced immune thrombotic thrombocytopenia after SARS-CoV-2 vaccination

Background: Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a newly defined prothrombotic disease that occurs after adenoviral vector vaccines against SARS-CoV-2. VITT is characterized by severe thrombocytopenia and thrombosis. Patients also have additional abnormalities on coagulation tests. The diagnosis of VITT depends on clinical findings and the detection of antiplatelet factor 4 antibodies.

Aims: In this study, we retrospectively evaluated the serological findings of patients referred to our platelet immunology laboratory for the diagnosis of VITT.

Methods: Serological investigations of clinical samples from patients suspected of VITT following a SARS-CoV2 vaccine were retrospectively investigated. Clinical and serological findings were retrieved from patient reports. The diagnosis of VITT was based on positive enzyme-linked immunosorbent assay (ELISA) and modified heparin induced platelet aggregation assay with PF4 (mod. HIPA). An optical density (OD) ≥0.500 was considered reactive in ELISA.

Results: A total of 301 samples from 262 patients (135 females, 127 males) were analyzed in our laboratory between March 2021 and Mai 2022. The median age of the patients was 51 years (range 14 to 87). Seventy-two samples were from our hospital and the remaining (n=229) samples were referred from other hospital or laboratories. Anti PF4/Heparin ELISA was positive in 51 patients (19.3%) and mod. HIPA was positive in 37 patients (14%). Of these, 3 had a negative ELISA. Among ELISA+ patients, OD IgG anti-PF4/heparin was higher in mod. HIPA+ patients than mod. HIPA- patients (2.44±0.86 vs 1.72±0.94; p=0.009). Platelet count was available in 62 patients. Only 53% (n=33) of these had a thrombocytopenia. Platelet count was negatively correlated with both D-dimer and IgG PF4/heparin antibody in ELISA (r=-0.33, p=0.015 and r=-0.37, p=0.002, respectively).

Conclusion(s): The diagnosis of VITT should be based on both clinical and laboratory findings. Lack of information on submissions from external hospitals and laboratories complicates the interpretation of serological tests.