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 is outlined as abnormality of renal structure and function during more than 3 months and associated with health complaints. An essential item of the guidelines bears on the classification of CKD, they are defined on the basis of glomerular filtration rates (GFR). Albuminuria is counted as another criterion of CKD. These main points are also listed in German S3 guidelines (DEGAM 2019).
Aspects on the determination of glomerular filtration rates (GFR) are discussed. There is consensus that GFR for clinical purpose is determined as eGFR (estimated glomerular filtration rate) by measuring endogenous creatinine (from the individual) and by using mathematical equations. Apart from measured serum creatinine, serum cystatin C is also useful as an alternative filtration marker. This holds especially true for children and in all cases in which creatinine measurements are suspected to give erreneous results. The estimation of eGFR is sufficient for clinical requirements. However, direct measurement of GFR (mGFR) with exogenous markers such as inulin remains the method of choice in defined conditions. The latter, however, is not a standard procedure in routine work though mGFR represents the real approach to disclose CKD.
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. Moreover, analytical interferences and critical anthropogenetic factors must be regarded which are prone to give wrong results. Underestimation and overestimation are inherent to the formulas. Critical limits are known and must be respected. Measurements of albuminuria and proteinuria will add further data on kidney function.