Laboratory testing for platelet factor 4 antibodies: differential utility for diagnosis/exclusion of heparin induced thrombocytopenia versus suspected vaccine induced thrombotic thrombocytopenia

Platelet factor 4 (PF4), a protein stored in the alpha-granules of platelets and released upon activation, forms cationic tetramers that bind with various polymeric anions, including heparin. Some individuals develop antibodies against PF4 in complex with heparin (PF4/H), which potentially lead to the onset of heparin induced thrombocytopenia (HIT). In some patients, this may cause activation and aggregation of platelets, promoting pathological thrombosis, in a process called heparin induced thrombocytopenia with thrombosis ('HITT'). Laboratories can assess for the presence of these antibodies using many PF4 antibody tests, including by enzyme linked immunosorbent assay (ELISA), latex immunoassay (LIA), chemiluminescence immunoassay (CLIA) and even rapid nanoparticle based lateral flow immunoassays. All these assays can identify such antibodies with high sensitivity, but methods may have variable specificity. For example, several studies have shown CLIA assays to have higher specificity to HITT than ELISA assays. Very recently, a new 'HITT-like' syndrome has been described in some individuals receiving adenovirus based COVID-19 (coronavirus disease 2019) vaccines. This condition has been given several names, including vaccine induced thrombotic thrombocytopenia (VITT) and thrombosis with thrombocytopenia syndrome (TTS), and also involves a mechanism mediated by antibodies formed against PF4. These antibodies can also be detected by PF4 antibody tests, but detection sensitivity appears to favour ELISA assays, with most other tests (including CLIA and LIA) not generally capable of detecting such antibodies. Additional functional assays assessing for PF4 mediated platelet activation may also be performed. The current review is focussed on laboratory testing for PF4 antibodies, in particular to distinguishing patterns in HITT versus VITT.