The relative low prevalence of abnormal and conditional TCD in the series compared to what described in patients living in USA or Europe (Adams and Nichols, 1990; Bernaudin et al., 2005) could be due to the low level of Hb as some authors reported that hypoxia induced by severe anemia increases blood flow velocity (Adams, 2007; Gulbis et al., 2005). This evidence concerns the gene GSTM1 and anemia (phenotype).