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A test of general renal function based on the measurement of urine osmolality has been applied to a group of normals, a group of control patients, and a series of patients suffering from renal disease of varying severity. The results obtained have been compared with the Van Slyke urea clearance tests carried out at the same time. In normal subjects and in the early stages of renal disease the two tests give reasonably similar results. In the advanced cases of renal disease, there is a marked discrepancy between the two assessments of renal function.

The majority of urine specimens, including the “morning specime”, can be shown to be mixtures of dilute and of concentrated urines. The differences in osmolality and the differences in concentration of these urines may be very considerable. The osmolality of a urine specimen after dehydration for 12–14 hours is a poor measure of the ability of the urinary system to elaborate a concentrated urine. If the urine flow rate is kept within the limits of 14 to 75 ml per hour, either by mild dehydration or by the smoking of tobacco, then there is a linear relationship between the water and the solute output in unit time.