BACKGROUND: Serum cystatin C (SCC)-based formulas and the newer creatinine formula (the Chronic Kidney Disease Epidemiology Collaboration formula (CKD-EPI)) were proposed as improved glomerular filtration rate (GFR) markers. The aim of our study was to compare serum creatinine (SCr)-based and SCC-based equations against (51)Cr-EDTA clearance in patients with mildly to moderately impaired kidney function. METHODS: 255 adult Caucasian patients with chronic kidney disease (GFR 89-30 ml/min/1.73 m(2)) were enrolled. In each patient, (51)Cr-EDTA clearance, SCr and SCC were determined. GFR was calculated using the Cockcroft-Gault (C&G), Modification of Diet in Renal Disease (MDRD), CKD-EPI formulas and simple cystatin C formula (SCCF) (100/SCC). RESULTS: The receiver-operating characteristic curve analysis (cut-off for GFR 60 ml/min/1.73 m(2)) showed that the SCCF had a higher diagnostic accuracy than C&G but not than MDRD or CKD-EPI formulas. The Bland-Altman analysis for the same cut-off value showed that creatinine formulas underestimated and SCCF overestimated the measured GFR. Analysis of ability to correctly predict a patient's GFR <60 or >60 ml/min/1.73 m(2) showed the higher ability for the SCCF compared to all creatinine-based formulas. CONCLUSION: Our results indicate that the SCCF is a reliable marker of GFR and comparable to creatinine formulas including the CKD-EPI formula.