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OBJECTIVES: To compare the accuracy of the two most popular creatinine clearance (CrCl) estimation formulae (Cockcroft-Gault (CG) and Modification Diet in Renal Disease (MDRD)) in older hospitalized patients. DESIGN: Prospective, cross-sectional, observational study. SETTING: Two hospital geriatric wards. PARTICIPANTS: Consecutive patients aged 70 and older with an indwelling urinary catheter for the purpose of care. MEASUREMENTS: CrCl was determined according to three methods: measured CrCl from plasma and urine creatinine and 24-hour urine volume, CG (CG-CrCl), and MDRD (MDRD-CrCl). Results were expressed as median and interquartile range (IQR). Moderate and severe renal impairment were defined as a CrCl between 30.0 and 59.9 mL/min and less than 30.0 mL/min, respectively. RESULTS: One hundred twenty-one patients were included (46% male). Mean age was 86.1±6.7 (range 72–100). Median measured CrCl was 43.8 mL/min (IQR 33.6–61.1 mL/min), CG-CrCl was 40.9 mL/min (IQR 31.0–52.6 mL/min), and MDRD-CrCl was 61.3 mL/min (IQR 49.4–77.0 mL/min). The biases of CG-CrCl and MDRD were −3.5±22.5 and 20.1±28.2, respectively ( P<.001). Misclassification of renal impairment (absent/moderate/severe) occurred in 33% of patients according to CG-CrCl, and concordance was mild to moderate (kappa=0.50). Misclassification occurred in 50% of patients according to MDRD-CrCl, and concordance was poor (kappa=0.33). Bias was significantly related to bed confinement for both formulae and to plasma creatinine for MDRD. CONCLUSION: In elderly hospitalized patients, CG slightly underestimates CrCl, and MDRD strongly overestimates it. CG gave a better prediction of measured CrCl than MDRD. [ABSTRACT FROM AUTHOR]