Table 1.

Case definition criteria for vaccine-induced immune thrombocytopenia and thrombosis (VITT): first described in 2021 as a new syndrome seen after first dose ChAdOx1.⁴

Case definition criteria	•Onset of symptoms 5–30 post vaccine (or up to 42 days if isolated DVT/PE) •Presence of thrombosis •Thrombocytopenia (platelet count <150 × 109/L) •D dimer >4000 mcg/mL (FEU) •Positive anti-PF4 Abs ELISA assay
Definite VITT (D)	Meets all five criteria
Probable (P)	•D dimer >4000 FEU but one criterion not fulfilled (Timing, Thrombosis, Thrombocytopenia, anti-PF4 Abs) •Or D dimer unknown or 2000–4000 FEU with all other criteria present
Possible (S)	•D dimer unknown or 2000–4000 FEU with one other criterion not fulfilled •Or two other criteria not fulfilled (Timing, Thrombosis, Thrombocytopenia, anti-PF4 Abs)
Unlikely (U)	•Platelet count <150 × 109/L without thrombosis with D dimer <2000 FEU •Or thrombosis with platelet count >150 × 109/L and D dimer <2000 FEU, regardless of anti-PF4 Ab result •And/or alternative diagnosis more likely