Purpose: Correct estimation of Glomerular filtration rate (GFR) is very important for an accurate clinical assessment of the kidney function. This study compares four GFR markers, a serum creatinine-based estimation using MDRD formula, Cystatin-C, Cr-51 EDTA 2 samples and 6 samples. Materials and Methods: Serum creatinine concentrations, Cystatin-C serum concentrations and Cr-51 EDTA clearance are measured in 43 patients who received or donated kidney. Results: The correlation coefficient between serum based estimated GFR (MDRD) and Cr-51 EDTA 6 samples was 0.817 (p<0.01). The correlation coefficient between Cystatin-C based GFR and EDTA 6 samples was 0.7322 (p<0.01). Regression analysis showed a statistically significant correlation between Cr-51 EDTA 2 samples and 6 samples (r=0.971, p<0.01). Mean value and ${pm}2SD$ for the difference between Cr-51 EDTA 2 samples and 6 samples were 4.7 mL/min and ${pm}9.3$ respectively. Conclusions: The estimation of two samples Cr-51 EDTA showed that the method can be simplified by reducing blood samples without losing its high accuracy.