Summary: In conformity with the guidelines of the Kidney Disease | Improving Global Outcomes (KDIGO) and the National Kidney Foundation (NKF), since the beginning of 2012, chronic renal disease (CKD) staging has been based on the cause, eGFR values and albuminuria. In the early stages of renal impairment, when the clinical picture is very poor, both laboratory markers – albuminuria and glomerular filtration rate – are particularly important. In recent years, the calculation of eGFR is based mainly on creatinine values, which have both advantages and disadvantages. In recent decades, the marker cystatin C has been studied as an alternative to creatinine. Since 2009, different formulas for calculating glomerular filtration (eGFR), including either creatinine or cystatin C alone, or both, have been proposed. In Bulgaria, the eGFR is mainly determined by the MDRD creatinine–based formula.