A simple method for correcting overestimated glomerular filtration rate in obese subjects evaluated by the Cockcroft and Gault formula: a comparison with 51Cr EDTA clearance.

Aim: The Cockcroft and Gault formula is a quick and reliable method for calculating creatinine clearance without a 24-hour urine collection (CG-cl). In obese subjects an excess of fat mass provokes a reduction in daily creatinine urine excretion per body kilo weight and is responsible for overestimated renal function when calculated by CG-cl. The aim of this study was to devise a simple correction method which could also make use of CG-cl in obese subjects.

Patients And Methods: In 52 subjects with a body mass index (BMI) > 25, renal function was assessed by simultaneously determining creatinine clearance using 24-hour urine collection (Cr-cl) and the CG-cl. The percentage difference between the 2 clearances (delta %) was correlated with BMI for each patient using simple linear regression analysis. The estimated regression model (delta% = 1.217 BMI-- 24.81) provided the following CG-cl correction formula for obese subjects: Corrected CG-cl = CG-cl (1.25 - 0.012 BMI). Its validity was evaluated in another group of 20 subjects with BMI > 25 by comparing the results obtained with Corrected CG-cl to those obtained by CG-cl and MDRD formula (MDRD-cl) using the clearance of 51Cr-EDTA (5 Cr-EDTA-cl) as the GFR measurement gold standard.

Results And Conclusion: Linear regression analysis of CG-cl, MDRD-cl and Corrected CG-cl compared to 5tCr-EDTA-cl (considered as the independent variable) resulted in the following determination coefficients (R2): 0.687; 0.818; 0.947, respectively. In conclusion, this formula can be considered a quick and reliable method for CG-cl correction in obese subjects.