PF4 and Thrombocytopenia: The American Society of Hematology (ASH) recommends that definitive diagnosis be made if all five of the following criteria are met: (i) COVID-19 vaccine 4 to 42 days prior to symptom onset, (ii) any venous or arterial thrombosis, (iii) thrombocytopenia (platelet count < 150 × 109/L), (iv) positive PF4 HIT ELISA and (v) markedly elevated D-dimer (>4 times upper limit of normal) [14].