Table 1. Case Definition Criteria for Vaccine-Induced Immune Thrombocytopenia and Thrombosis (VITT), According to an Expert Hematology Panel.^*.

Type of VITT	Description
Definite VITT	All five of the following criteria: Onset of symptoms 5–30 days after vaccination against SARS-CoV-2 (or ≤42 days in patients with isolated deep-vein thrombosis or pulmonary embolism) Presence of thrombosis Thrombocytopenia (platelet count <150,000 per cubic millimeter) d-dimer level >4000 FEU Positive anti-PF4 antibodies on ELISA
Probable VITT	d-dimer level >4000 FEU but one criterion not met (timing, thrombosis, thrombocytopenia, or anti-PF4 antibodies) or d-dimer level unknown or 2000–4000 FEU and all other criteria met
Possible VITT	d-dimer level unknown or 2000–4000 FEU with one other criterion not met, or two other criteria not met (timing, thrombosis, thrombocytopenia, or anti-PF4 antibodies)
Unlikely VITT	Platelet count <150,000 per cubic millimeter without thrombosis with d-dimer level <2000 FEU, or thrombosis with platelet count >150,000 per cubic millimeter and d-dimer level <2000 FEU, regardless of anti-PF4 antibody result, and alternative diagnosis more likely

^*ELISA denotes enzyme-linked immunosorbent assay, FEU fibrinogen-equivalent unit, PF4 platelet factor 4, and SARS-CoV-2 severe acute respiratory syndrome coronavirus 2.