Sirs, We have read with interest a number of contributions in recent issues of the journal [1, 2, 3] pointing out that cystatin C remains a current issue. We therefore present our own results concerning this problem. Cystatin C (cysC) is low molecular protein which is recognized as a good indicator of glomerular filtration rate (GFR) by most authors [4]. Unlike creatinine, cysC has constant reference range after first year of life [5, 6, 7]. Its serum concentration does not depend on height, weight, or muscular mass [5]. CysC is more sensitive than creatinine in determining mild reduction in GFR [8, 9, 10]. A number of authors have presented the results indicating efficacy of cysC in assessing renal function, such as in transplanted patients and those with malignant hematological or endocrinological conditions [11, 12, 13, 14]. The efficacy of cysC as a marker of GFR in children was the object of our research. We examined 91 children aged 2–19 years. These were divided into three groups: group I, controls (n=46); group II, those with chronic nonterminal renal failure (n=20); and group III, those on hemodialysis (n=25). Serum creatinine and cysC were determined in all groups and cysC and creatinine clearance (Ccr) in groups I and II. CysC was measured by nephelometric assey using the N-latex cystatin c kit (Dade Behring). Statistical analysis was performed by standard computer programs. In groups I and II we found a significant effect of age, weight, height, and body surface area on serum creatinine level. All none of these factors was there an effect on cysC levels. In group III there was a correlation between serum creatinine and cysC with weight, height, and body surface area and in the case of creatinine also with body mass index. Following the average values of creatinine and cysC, we noted that the average values of cysC first demonstrated a mild increase in Ccr of 81–90 ml/min per 1.73 m. In the terminal renal failure and in dialysis group, cysC did not show advantage over creatinine. There was a very significant correlation between serum creatinine and Ccr levels. Pearson’s correlation coefficient for reciprocal creatinine and Ccr was r=0.83 (95% CI 0.72–0.89, P<0.0001). The association between the values for serum cysC (1/cys) and Ccr were even more marked. Pearson’s correlation coefficient was higher (r=0.92, 95% CI 0.86– 0.95, P<0,0001), which shows that cysC is probably a better indicator of renal function. However, statistical analysis of our sample did not show that cysC is a statistically much better indicator of renal function than creatinine is. Receiver operating characteristic analyses showed good diagnostic characteristics of both methods for estimating of renal function. However, the shape of the curve for cysC demonstrated better precision of this method. The area under receiver operating characteristic curve (AUC) for 1/cys was 0.976€0.02 (95% CI 0.943– 1.00) while that for 1/Cr was somewhat smaller (0.903€0.05, 95% CI 0.801–1.00), which means that serum cysC is a more accurate diagnostic procedure for estimating of renal function than creatinine. However, a discrepancy of 0.073 is not statistically significant (P=0.13). When determining creatinine, for the maximum level of efficiency which was 91.07%, the sensitivity was 84%, specificity 96.80%, positive predictive value 95.45%, and the negative predictive value 85.71%. When determining cysC for the maximum level of efficiency, which was 94.64%, the sensitivity was 92%, specificity 96.8%, positive predictive value 95.83%, and negative predictive value 90.91%. The results of our research show that there is no significant effect of age or anthropometric measures on the serum level of the cysC in healthy children or in children with nonterminal chronic kidney disease. We also conclude that measuring cysC increases the detection of S. Pavicevic ()) Institute for Diseases of Children, 81000 Podgorica, Serbia and Montenegro e-mail: snezanaisrdjan@cg.yu Tel.: +381-81-244432 Fax: +381-81-412528