GSTM1 and anemia (phenotype): With regard to the Hb cutoff point to classify as positive for anemia, we found 12 studies [20,21,22,24,25,26,27,28,29,31,32,33] used the correction for altitude recommended by WHO [4]; however, 6 studies [20,21,23,27,29,32] evaluated Hb without using this correction factor (Table 2 and Table 3).