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 (418 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.