We undertook a pilot randomized controlled trial (RCT) to assess the feasibility of Optimizing Transfusion Threshold in Critically-ill Children with Anaemia (OpTTICCA) which will be a large international non-inferiority RCT addressing the efficacy and safety of Hb thresholds of 70 g/L not only in hemodynamically stable children, but in almost all PICU patients with a Hb concentration ≤ 95 g/L. Here, GSTM1 is linked to anemia (phenotype).