The presented material raises the most important question in practical nephrology – how to correctly assess kidney function to understand the prognosis and duration of the predialysis period in a particular patient with chronic kidney disease (CKD)? From the standpoint of onto- and phylogenesis, the hierarchy of kidney functions was assessed. It is noted that the existing approaches to such an assessment are convenient for practice, but do not sufficiently take into account individual characteristics and are devoid of a load component that can show kidney reserves. The authors focused on the functional renal reserve (FRR) and the possibility of its detection. The above technique with 0.45 % saline allows revealing the true functional capabilities of the kidneys and understanding the patient's capabilities in the pre-dialysis period. Conclusions are drawn about the convenient use of the KDIGO scale, a more individual approach when using the QxMD calculator, and the possibility of a personalized approach when assessing the FRR.