In addition, the American Society of Hematology declared the five mandatory diagnostic criteria for this condition to be as follows: COVID-19 vaccination occurring 4–42 days prior to symptom onset, any venous or arterial thrombosis (often cerebral or abdominal), thrombocytopenia (platelet count < 150 × 109/L), positive PF4 “HIT” ELISA findings, and markedly elevated D-dimer levels (> four times of the upper limit of normal) [54]. Here, PF4 is linked to Thrombocytopenia.