We aimed to investigate the performance of various creatinine based glomerular filtration rate estimation equations that were widely used in clinical practice in Turkey and calculate a correction coefficient to obtain a better estimate using the isotope dilution mass spectrometry (IDMS)-traceable Modification of the Diet in Renal Disease (MDRD) formula. This cross-sectional study included adult (>18 years) outpatients and in patients with chronic kidney disease as well as healthy volunteers. Iohexol clearance was measured and the precisions and bias of the various estimation equations were calculated. A correction coefficient for the IDMS-traceable MDRD was also calculated. A total of 229 (113 male/116 female; mean age 53.9 ± 14.4 years) subjects were examined. A median iohexol clearance of 39.21 mL/min/1.73 m(2) (range: 6.01-168.47 mL/min/1.73 m(2)) was found. Bias and random error for the IDMS-traceable MDRD equation were 11.33 ± 8.97 mL/min/1.73 m(2) and 14.21 mL/min/1.73 m(2), respectively. MDRD formula seems to provide the best estimates. To obtain the best agreement with iohexol clearance, a correction factor of 0.804 must be introduced to IDMS-traceable MDRD equation for our study population.