Anaemia in children less than 5 years old is classified by the World Health Organization (WHO) into three categories, according to Haemoglobin (Hb) concentration in blood: severe (Hb ≤ 5 g/dl), moderate (5 <Hb < 8 g/dl) and mild (8 ≤ Hb < 11 g/dl). This evidence concerns the gene GSTM1 and anemia (phenotype).