Tests for renal function are of two types: those that determine the power of the kidney to dilute and concentrate the urine and those that measure the ability of the kidney to eliminate the renal excretory products. Those of the first group, which includes the various tests for specific gravity, are well established and adequate; those of the second are not completely satisfactory. Beside the use of the dye tests, which are not specific, the measure of renal excretion usually is attempted by a determination of the amount of urea or nonprotein nitrogen in the blood. These estimations demonstrate how much of these substances is actually retained, but not the degree of the impairment of renal function. This is especially true when the figures for the blood chemistry determinations are low; it is not then certain whether renal function is normal or impaired, since the level of the nonprotein nitrogen
MOSENTHAL HO, BRUGER M. THE UREA RATIO AS A MEASURE OF RENAL FUNCTION. Arch Intern Med (Chic). 1935;55(3):411–419. doi:10.1001/archinte.1935.00160210064006
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