Because a severely anaemic patient (e.g., Hb = 6.0 g/dl) is more likely to receive one or more RBC units than a mildly anaemic patient (e.g., Hb = 12.0 g/dl), we also evaluated the influence of Hb level as a continuous variable (instead of the binary presence or absence of anaemia) among other possible confounders using ordinal regression. Here, GSTM1 is linked to anemia.