In the absence of inflammation, serum ferritin is widely agreed to be the most specific non-invasive biochemical test to diagnose iron deficiency and is an accurate reflection of total body iron stores.19 Assaying ferritin levels is useful because a reduction in this marker occurs before detectable reductions in Hb, serum iron levels or erythrocyte size, which traditionally indicate the presence of anaemia at more severe stages of iron deficiency. This evidence concerns the gene GSTM1 and anemia.