Estimating Glomerular Filtration Rate (GFR) in Obese Subjects (#237)
Introduction: Renal function can be estimated by formulae that incorporate patients’ age, gender, creatinine and, sometimes, weight. The accuracy of this estimate is optimal when lean body mass (LBM) is substituted for actual weight in the Cockcroft-Gault (CCG) formula. LBM measurement is not easily performed in hospital, especially in the obese inpatient. The aim was to determine an acceptable bedside method for GFR estimation in obese patients.
Methods: In a series of seventy obese Caucasian outpatients, anthropometric and other characteristics were collected including LBM by impedance analysis. With CCGLBM as a reference, estimates of GFR were compared using Bland-Altman plots, correlation analysis and t-testing. All estimates were more easily generated at the bedside than the CCGLBM. Data are expressed as mean ± SD.
Results: The patients (74% female) were aged 43 ± 13 years with BMI 47 ± 8 kg/m2, height 168 ± 9 cm, body fat 52 ± 6 % and serum creatinine 70 ± 27 µM. GFR estimated by the CCGLBM was 98 ± 35 ml/min. GFR estimates using the other formulae ranged from 100 ± 24 to 221 ± 88 ml/min. The CCG formula incorporating actual body weight overestimated the GFR by >120 ml/min (p<0.001) with a significant fixed proportional bias (p<0.001). For GFRs between 20 and 180 ml/min, using ideal body weight (CCGIBW) made no significant difference to the CCGLBM estimate. The Modified Diet in Renal Disease (MDRD) and Chronic Kidney Disease Epidemiology Collaboration (CKDEPI) formulae performed as well as the CCGLBM in estimating GFR but with proportional bias (p<0.001). Remaining formulae demonstrated significant fixed (p<0.001) and proportional inaccuracies (p<0.001).
Conclusion: The CCGIBW is equivalent to an accepted best estimator of GFR in the obese. Considering the ease of its calculation at the bedside, the CCGIBW has a role in GFR estimation of obese inpatients.