Since the fractional fall in hemoglobin is correlated positively with the starting Hb, the metric chosen were both development of severe anaemia (Hb < 5 g/dL) and a composite measure of a high fractional fall (> 25%) to an absolute Hb below 5 g/dL, at which level the risk of mortality rises significantly [28]. This evidence concerns the gene GSTM1 and anemia (phenotype).