The endogenous creatinine clearance in normal subjects.

In normal subjects the glomerular filtration rate is relatively constant under standard conditions, and there is little or no change when the urine flow 'aries. It follows from this that the excretion rate of substances, which are filtered at the glomerulus and neither reabsorbed from nor secreted into the tubules, is constant. It is these substances which can be used to measure the filtration rate. There is considerable evidence that endogenous creatinine behaves in this way, and this is supported by the data presented in this paper, which show that its excretion is very constant in spite of considerable variations in urine flow, only decreasing slightly at low urine volumes.