Demystifying autoimmune HIT: what it is, when to test, and how to treat.

Antibodies to platelet factor 4 (PF4) have been primarily linked to classical heparin-induced thrombocytopenia (cHIT). However, during the rollout of the COVID-19 vaccine program a new condition, vaccine-induced thrombocytopenia and thrombosis (VITT), was identified, related to adenoviral-based COVID-19 vaccines. The differences between these 2 conditions, both clinically and in laboratory testing, set the scene for the development of a new rapid anti-PF4 assay that is not linked with heparin (as relevant for cHIT). Concurrently, there has been a reassessment of those cases described as autoimmune HIT. Such scenarios do not follow cHIT, but there is now a clearer differentiation of heparin-dependent and heparin-independent anti-PF4 conditions. The importance of this distinction is the identification of heparin-independent anti-PF4 antibodies in a new subgroup termed VITT-like disorder. Cases appear to be rare, precipitated by infection and in a proportion of cases, orthopaedic surgery, but are associated with high mortality and the need for a different treatment pathway, which includes immunomodulation therapy.