Possible sources of heterogeneity were explored by sensitivity and, if possible, by subgroup analyses and could be related to study design, duration of follow-up, administered dose of pentoxifylline, population characteristics (e.g. severity of CKD, dialysis vs. conservative treatment), baseline Hb levels and iron status, concomitant treatment with ESAs or iron supplements. This evidence concerns the gene GSTM1 and chronic kidney disease.