Background: The Cockcroft-Gault (CG) and simplified Modification of Diet in Renal Disease (MDRD) formulas are the most widely used estimates of renal function. The influence of age and body mass index (BMI) on the performance of these equations was analyzed in 850 subjects with serum creatinine levels less than 1.5 mg/dL (<133 micromol/L).
Methods: Glomerular filtration rate (GFR) was measured as urinary clearance of continuously infused technetium Tc 99m-labeled diethylene triaminopentaacetic acid. Performance was assessed as bias, precision, and accuracy.
Results: In the total population, the CG and MDRD calculations based on enzymatic measurement of serum creatinine (which is constantly less than that obtained by using the alkaline picrate [Jaffé] method) significantly underestimated GFR by 4.9 and 12.4 mL/min/1.73 m2 (0.08 and 0.21 mL/s/1.73 m2), respectively. In patients 65 years and older, underestimation by means of the CG formula was enhanced, whereas that by means of the MDRD formula was blunted, compared with the group younger than 65 years (-11.3 versus -3.7 mL/min/1.73 m2 [-0.19 versus -0.06 mL/s/1.73 m2] for CG and -3.7 versus -14.0 mL/min/1.73 m2 [-0.06 versus -0.23 mL/s/1.73 m2] for MDRD). GFR was underestimated to a large extent by means of the MDRD equation irrespective of BMI. Conversely, the underestimation by means of the CG formula found in lean people (-13.0 mL/min/1.73 m2 [-0.22 mL/s/1.73 m2]) was blunted in overweight people (BMI, 25 to 30 kg/m2) and reversed to overestimation (+10.1 mL/min/1.73 m2 [+0.17 mL/s/1.73 m2]) in obese subjects (BMI > 30 kg/m2).
Conclusion: As suggested by estimations obtained using enzymatic serum creatinine measurement, the MDRD equation may be the estimation of choice in elderly patients, whereas the CG estimate is preferable in subjects younger than 65 years. Nevertheless, when obesity is present, no reliable estimation can be obtained by using the CG or MDRD formula.