Quantification of excretory renal function and urinary protein excretion by determination of body cell mass using bioimpedance analysis

BACKGROUND: Creatinine clearance (CrCl) based on 24 h urine collection is an established method to determine glomerular filtration rate (GFR). However, its measurement is cumbersome and the results are frequently inaccurate. The aim of this study was to develop an alternative method to predict CrCl and urinary protein excretion based on plasma creatinine and the quantification of muscle mass through bioimpedance analysis (BIA). METHODS: In 91 individuals with normal and impaired renal function CrCl was measured from 24 h urine excretion and plasma creatinine concentration. A model to predict 24 h-creatininuria was developed from various measurements assessing muscle mass such as body cell mass (BCM) and fat free mass (FFM) obtained by BIA, skinfold caliper and other techniques (training group, N = 60). Multivariate regression analysis was performed to predict 24 h-creatininuria and to calculate CrCl. A validation group (N = 31) served to compare predicted and measured CrCl. RESULTS: Overall (accuracy, bias, precision, correlation) the new BIA based prediction model performed substantially better compared with measured CrCl (P(15) = 87 %, bias = 0, IQR of differences = 7.9 mL/min/1.73 m(2), R = 0.972) versus established estimation formulas such as the 4vMDRD (P(15) = 26 %, bias = -8.3 mL/min/1.73 m(2), IQR = 13.7 mL/min/1.73 m(2), R = 0.935), CKD-EPI (P(15) = 29 %, bias = -7.0 mL/min/1.73 m(2), IQR = 12.1 mL/min/1.73 m(2), R = 0.932, Cockcroft-Gault equations (P(15) = 55 %, bias = -4.4 mL/min/1.73 m(2), IQR = 9.0 mL/min/1.73 m(2), R = 0.920). The superiority of the new method over established prediction formulas was most obvious in a subgroup of individuals with BMI > 30 kg/m(2) and in a subgroup with CrCl > 60 mL/min/1.73 m(2). Moreover, 24 h urinary protein excretion could be estimated accurately by normalization with 24 h-creatininuria derived from BIA based BCM. CONCLUSION: Prediction of CrCl based on estimated urinary creatinine excretion determined from measurement of BCM by BIA technique is both accurate and convenient to quantify renal function in normal and diseased states. This new method may become particularly helpful for the evaluation of patients with borderline renal insufficiency and/or with abnormal body composition.