This GFR cutoff was chosen based on Mercadal et al., who demonstrated that the physiologic response to anemia is somewhat preserved in patients with GFRs greater than 30 mL/min/1.73 m2 and that there is no significant correlation between the EPO level and GFR above this cutoff.[14] Thus, in our study we focused on patients with severe CKD since patients with GFRs greater than 30 mL/min/1.73 m2 were unlikely to have CKD as the sole contributor to the anemia. This evidence concerns the gene EPO and anemia.