PF4 and pulmonary embolism: VITT is characterized by both arterial and venous thrombotic events in unusual sites, such as splanchnic veins and cerebral venous sinus, accompanied by low platelets count (<150 × 103/mm3), markedly elevated D-dimer (>4 times upper limit of normal), and positive Platelet Factor 4 (PF4) Heparin-Induced Thrombocytopenia (HIT) Enzyme-Linked Immuno-Sorbent Assay (ELISA), 4–42 days after vaccine administration; pulmonary embolism and disseminated intravascular coagulation may also occur [25].