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Introduction: We measured serum creatinine concentrations in 17 male athletes of the Italian national rugby team., Methods: Blood was obtained before the start of training and during the competitive season. Serum creatinine level was measured by Jaffé reaction at 4 time points during the season, with a formal measure of creatinine clearance in midseason., Results: The values of estimated glomerular filtration rate (eGFR) calculated with the Cockcroft-Gault (CG) equation were higher than those calculated with the Modification of Diet in Renal Disease (MDRD) Study formula (p<0.001). This difference was significantly decreased but still present when the MDRD formula was corrected for body surface area (BSA). When compared with measured creatinine clearance (CrCl), the MDRD underestimates the CrCl by 51 ml/min (95% confidence interval [95% CI], 36-67 ml/min, p<0.0001). When corrected for BSA, this difference falls to 27 ml/min (95% CI, 13-44 ml/min, p=0.001). The CG eGFR gave a better estimate of CrCl, differing by 1 ml/min (95% CI, -16 to +17 ml/min, p=NS)., Conclusions: The MDRD formula underestimates the CrCl in rugby players, even when corrected for BSA. Conversely, the CG formula more closely approximates the actual CrCl measurement. The equations to estimate GFR should be used with caution in subjects having atypical anthropometric characteristics.