KDIGO 2012 guidelines (KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease) cover the field of definition, classification and management of chronic kidney disease (CKD). CKD classification is mainly based on glomerular filtration rates (GFR). There is consensus that GFR for clinical purpose is determined as eGFR (estimated glomerular filtration rate) by measuring endogenous creatinine and by using mathematical equations. The estimation of eGFR is sufficient for clinical requirements. MDRD formulas and the equation of the CKD-EPI working group are widely used for eGFR calculations. Reproducibility of results, however, depends to a great extent on standardisation of creatinine and cystatin C assay sytems. However, underestimation and overestimation are inherent to the formulas. Moreover, analytical interferences and critical anthropogenetic factors must be regarded.