Extracellular fluid volume and glomerular filtration rate

Abstract

The slope-only technique for measuring glomerular filtration rate (GFR) relies on extracellular fluid volume (ECV) remaining within narrow limits. Although this requirement is met in healthy individuals, ECV may deviate or vary more in patients with abnormal renal function. We examined the correlation between surface area-scaled ECV and GFR, and their coefficients of variation (CVs), measured from simultaneous, multisample clearances of 51Cr-EDTA and iohexol in 20 healthy volunteers and 60 patients with a range of renal functions. We also compared scaled GFR and ECV, and their CVs, measured from three-sample, slope-intercept clearance of 51Cr-EDTA in 921 patients routinely referred for GFR measurement. In the 80 participants undergoing multisample, dual-indicator clearance, there was no correlation between GFR measured with one indicator and ECV measured with the other. CVs of GFR in the 60 patients were 48.1 and 44.6% for 51Cr-EDTA and iohexol, respectively, but the CVs of ECV were only 12.3 and 15.4%. These differences were less marked in the healthy participants with corresponding CVs of 13.9 and 14.9% for GFR, and 11.7 and 12.2% for ECV. There was no correlation between scaled GFR and ECV in patients having slope-intercept clearance; CVs of GFR and ECV were 32.4 and 17.8%, respectively. In unselected patient populations, there is no correlation between GFR and ECV. The CV of ECV is slightly higher in patients than healthy individuals but, in both, is less than the CV of GFR. These data do not detract from the use of slope-only GFR.