Renal function has a significant impact on early mortality in patients with cirrhosis. However, creatinine and creatinine-based equations are inaccurate markers of renal function in cirrhosis. The aim of this study was to reassess correlations between creatinine-based equations and measured glomerular filtration rate (GFR) and to investigate the impact of inaccuracies on the Model for End-Stage Liver Disease (MELD) score. GFR was measured using iohexol clearance and calculated with creatinine-based equations in 157 patients with cirrhosis during pretransplant evaluation. We compared the accuracy of creatinine to that of true GFR in a prognostic score also including bilirubin and the international normalized ratio. In patients with creatinine below 1 mg/dL, true GFR ranged from 34-163 mL/minute/1.73 m2. Cockcroft and Modification of Diet in Renal Disease (MDRD) significantly overestimated true GFR. On multivariate analysis, younger age and ascites were significantly correlated with the overestimation of true GFR by 20% or more. Body mass index was an independent risk factor of overestimation of GFR with Cockcroft but not with MDRD. The accuracy of a prognostic score combining bilirubin, international normalized ratio, and true GFR was superior to that of MELD, whether creatinine was rounded to 1 mg/dL when lower than 1 mg/dL or not (c-statistic of 0.8 versus 0.75 and 0.73, respectively). Creatinine-based formulas overestimate true GFR, especially in patients younger than 50 years or with ascites. In patients with serum creatinine below 1 mg/dL, the spectrum of true GFR is large. True GFR seems to have a better prognostic value than creatinine and creatinine-based equations. Specific equations are needed in patients with cirrhosis to improve prognostic scores. Liver Transpl 16:1169–1177, 2010. © 2010 AASLD.