Renal Physiology in Infants and Children: I. Method for Estimation of Glomerular Filtration Rate

Abstract

The more exact methods for estimation of kidney function have been quite successfully employed in older children, 1 but they have not been very widely applicable to infants because of the obvious difficulties in obtaining complete urine specimens. Schoenthal and his coworkers 2 studied the urea clearance in 9 normal infants ranging in age from 2 to 11 1/2 months and concluded that the urea clearance corrected for surface area agreed with the values observed by Van Slyke and his coworkers for older children and adults, which indicated to them that renal function measured by the ability to excrete urea is as well developed in infants as in later life. Their studies did not include, however, the investigation of renal function during very early postnatal life. The method to be described here originated in an attempt to study the renal physiology during the newborn period. The single injection inulin clearance test proposed by Alving and Miller 3 seemed especially applicable to this problem. Inulin clearances seemed preferable to urea clearances for this study since it has been well established that the inulin clearance is at the level of glomerular filtration and that the filtration rate is fairly constant at low and high rates of urine formation. 4

Alving and Miller have shown that after the intravenous injection of inulin (10 g in adults) the plasma inulin concentration, plotted logarithmically against time, decreased at first in a curvilinear manner, later falling, however, in a linear or almost linear relationship with time. The curvilinear relationship marks, presumably, the period of equilibration between the blood and plasma and the extracellular fluids. The straight line relationship follows because once equilibrium has been established the rate of fall of the plasma inulin level is determined by the rate at which inulin is cleared from the plasma by the kidneys.