Platelet-activating anti-PF4 antibodies mimic VITT antibodies in an unvaccinated patient with monoclonal gammopathy

Transient prothrombotic disorders caused by plateletactivating antibodies against platelet factor 4 (PF4) include heparin-induced thrombocytopenia (HIT), spontaneous HIT syndrome,1 and, most recently, vaccine-induced immune thrombotic thrombocytopenia (VITT).2 Here, we identified prothrombotic, platelet-activating anti-PF4 antibodies, not associated with heparin treatment, in a patient with monoclonal gammopathy that resulted in a chronic hypercoagulability state.