PF4 and Venous thrombosis: TTS is diagnosed if four criteria are met contemporaneously: (1) COVID-19 vaccine administered from 4 to 30 days before the event; (2) venous or arterial thrombosis (often cerebral or splanchnic); (3) thrombocytopenia (<150×109/L or <50% from baseline); and (4) positive PF4 HIT on ELISA in the absence of heparin (10, 29, 30).