Assessment of glomerular filtration rate by formulas in obese patients: What is the best choice?

Background: The equations routinely used to calculate the glomerular filtration rate (GFR) have not been validated in obese patients.

Materials And Methods: This cross-sectional study evaluated 7 formulas used to estimate GFR, analyzing the effect of using ideal body weight (IBW) through the formula IBW = desirable BMI × (height), using the calculated body surface area (BSA) of each patient through the Dubois and Dubois formula. Bias, accuracy, and receiver operating characteristic curve were calculated. The criterion standard was the direct measurement of GFR by 24-hour urine creatinine clearance.

Results: Forty-five obese patients were evaluated (grade I obesity 48.89%). The formula that showed greater accuracy in grade I obesity was Salazar (% error = 2.30 ± 33.92), followed by Cockcroft-Gault (% error = -2.84 ± 32.76). In patients with grade II and III obesity, the most accurate formula was Chronic Kidney Disease Epidemiology (CKD-EPI) (% error = 3.84 ± 41.79), followed by the Modification of Diet in Renal Disease (MDRD) formula (% error = 4 ± 38.43). When using IBW in the Cockcroft-Gault, Sobh, and Salazar formulas, the mean GFR was closer to the criterion standard and showed an increase in the Pearson correlation. Of these 3, the one with the best performance in grade II and III obesity was Cockcroft-Gault (% error = 8.90 ± 42.96).

Conclusion: The results demonstrate that in this sample, the use of IBW improved Cockcroft-Gault performance. Cockcroft-Gault BSA showed lower performance when compared to Cockcroft-Gault. In grade II and III obesity, CKD-EPI and MDRD showed the best accuracy.