GSTM1 and anemia: Aplasia or haemolysis may be precipitated by another illness/infection in patients with sickle cell disease. Acutely severe anaemia is defined as Hb < 5 g/dl or a recent acute drop in HB > 2 g/dl below the individual’s steady state value. If no steady state value is available, it can be detected by the presence of acutely symptomatic anaemia (i.e. tachycardia, cardiac failure, shock).