The authors determined cystatin C serum levels and 24-hour creatinine clearance (GF cre a) to evaluate the clinical usefulness for glomerular filtration rate (GFR) estimation. Cystatin C clearance (GF cys) was calculated by using a coefficient published in the literature. 78 patients with various kidney diseases were included in the study, nobody had haemodialysis. The patients were divided into two group according to their age – group A, 45 patients, age 19 – 50 years, groups B, 33 patients, age 51 – 78 years. Serum cystatin C was determinated by using the particle–enhanced-turbidimetric method DAKO, 24-hour creatinine clearance was assessed by the usual procedure. Cystatin C levels in group A were 0.7 – 4.54 mg/l (reference intervals 0.63 – 1.33) GF creatinine 0.29 – 2.94 ml/s (1.25–2.30) and GF cystatin 0.20 – 3.8 ml/s, in the group B cystatin C levels were 0.0 – 4.38 mg/l (0.74–1.55), GF creatinine 0.0–2.0 ml/s (1.15–2.0) and GF cystatin 0.0 – 3.3 ml/s. GF creatinine correlated inversely to serum cystatin C, r = –0.766, p < 0.001 in the group A and r = –0.697, p < 0.001 in the group B. These data suggest that cystatin C in serum could be useful in estimating GFR in clinical practice and that mild- ly elevated cystatin C in serum could indicate a mild GFR reduction, still with normal values of GF creatinine. Cystatin C serum levels could be influenced by interferences of several substances which are to be assessed.

        Key words: cystatin C, GF creatinine, GFR evaluation, impaired renal function.