Reproducibility of GFR measured by chromium-51-EDTA and iohexol.

We read with interest the article published by Bird et al about the reproducibility of measured GFR by iohexol and Cr-51-EDTA [ 1 ]. We would like to make one comment and to share our own results on this fundamental (although understudied) topic. As we already pointed out in a NDT editorial, we share the same reserves about the opportunity to index glomerular filtration rate (GFR) with body surface area [ 2 ]. Indexing GFR with extracellular fluid volume (ECV) may be viewed as more logical as one of the major functions of the kidney is to regulate ECV. However, the fact that reproducibility of the GFR is better when indexed with ECV than when indexed with BSA is not the definite proof to use ECV indexation. The only proof would be to have an absolute correlation between GFR and ECV and that this relationship would totally disappear between indexed GFR and ECV [ 2 ]. For these reasons, we think that reproducibility of GFR measurement must be calculated from absolute, non-indexed GFR. Can Bird et al give the reproducibility of their non-indexed GFR?