Relationships among Characters Observed in Urea Clearance Tests

Abstract

I N LABORATORIES of clinical pathology the observation usually made to measure efficiency of kidney action is that known as the van Slyke urea test. Failure of renal function results eventually in increased concentration of urea in the blood; the objective of the test is to provide a measure of partial failure before the onset of that consequence. It has been supposed that this can be obtained by observing the rate of urea excretion in urine. Following observations (under somewhat limited and special conditions) that urea excretion is approximately proportional to concentration in the blood, the measure adopted for test purposes has been the ratio of urea excreted per minute/urea concentration in blood usually described as measuring the volume of blood cleared of urea per minute, although of course no blood is actually cleared. A second consideration which the van Slyke test was devised to meet is that rate of urea excretion depends also on the rate of water excretion, particularly when this is slow. It was observed that rate of urea excretion is maintained nearly constant when rate of urine excretion exceeds about 2 ml. per minute. Consequently maximum urea clearance was defined as