However, no relationship has been found between the serum levels of uremic toxins, including IS, and hemoglobin (Hb) levels or other anemia parameters in CKD patients in most studies [48,49,50], but not in all [51,52]; the association was lost in the multivariate analysis, while in one negative study, only in the subset of non-anuric peritoneal dialysis patients a relationship was found between IS and Hb levels [48]. This evidence concerns the gene GSTM1 and chronic kidney disease.