Extracellular volume and glomerular filtration rate in children with chronic kidney disease.

BACKGROUND AND OBJECTIVES Extracellular volume (ECV) is the fluid contained in all noncellular compartments of the body and is a quantity tightly controlled by the kidney. Thus, there is a strong link between ECV and kidney function. DESIGN, SETTING, PARTICIPANTS & MEASUREMENTS The Chronic Kidney Disease in Children (CKiD) study uses injected iohexol to obtain direct measures of GFR. Direct calculation of ECV was viable from GFR studies using descriptors of the disappearance curves. Using linear regression methods on the log-transformed variables, markers of size (height and weight) and biomarkers of kidney disease (serum creatinine, blood urea nitrogen, and cystatin C) were assessed for their relationships with ECV normalized to body weight (ECV/wt). The relationship to hypertension (systolic BP >95th percentile for age, sex, and height) was also assessed. RESULTS Data from 790 iohexol studies with medians for GFR=43.4 ml/min per 1.73 m(2), weight=35 kg, and height=1.4 m were used. The median ECV was 8.6 L, and the median ECV/wt was 0.23 L/kg. ECV was found to be a function of height (m) and weight (kg) according to the relationship ECV=√weight×height. Biomarkers of kidney disease yielded significant relationships with ECV/wt, but the strength of association was small. No significant association between ECV/wt and hypertension was found. CONCLUSIONS ECV has relevance to studies of chronic kidney disease, is related to biomarkers, and can be easily estimated from the square root of weight and height.