Age, stage and biomarkers for the definition of CKD: a construction in progress.

The international recommendations of the Kidney Disease: Improving Global Outcomes (KDIGO) to define chronic kidney disease (CKD) and classify patients in CKD stages are discussed in an opinion paper published in this issue of the journal. In this counterpoint, we will review some questions and criticisms raised by the authors to provide further contribution on the issue. In particular, we would like to discuss the age issue in the definition of CKD, the validity of the KDIGO staging, the validity of creatinine-based equations for the estimation of the glomerular filtration rate (GFR), as well as the clinical value of cystatin C and the epidemiological rather than clinical nature of the arguments proposed to justify recommendations in the KDIGO guidelines.