PF4 and deep vein thrombosis: According to the review published by the American Society of Hematology (ASH), five criteria were given to definitively diagnose TTS: 1) COVID-19 vaccination 4 to 42 days before symptom onset, 2) any venous or arterial thrombosis (often cerebral or abdominal), 3) thrombocytopenia, 4) positive for PF4 ‘HIT’ (heparin-induced thrombocytopenia) ELISA, 5) markedly elevated D-dimer (> 4 times the upper limit of normal) (13).