Abstract
The urea contents of urine and blood are compared in a colorimeter in such a manner that a single reading gives directly the percentage of average normal renal function in terms of the blood urea clearance. 1 The urine is first diluted to such an extent that if the clearance, either standard or maximum, is the average for a normal subject, the urea concentrations in blood and diluted urine will be equal. The urea in both blood and urine is converted into ammonia with urease; proteins and other interfering substances are removed, and the ammonia contents of the 2 filtrates are compared colorimetrically.
No standard solutions are required, because the blood is compared directly with the urine.
The number of times the urine must be diluted for comparison with the blood is found by reference to the curve of Fig. 1, computed as follows: The average maximum clearance (for urine volumes over 2 cc. per minute) is 75. The maximum clearance is calculated as Cm = UV/B where V is reckoned in cc. of urine per minute, B is blood urea concentration, and U is urine urea concentration. When C m has the average normal value of 75, the ratio U/B is calculated therefore as U/B = 75/V. When V is expressed in cc. of urine per hour, the calculation changes to U/B = 4500/V. The ratio U/B is the number of times a portion of the urine must be diluted to bring its urea concentration down to that of the blood, when the clearance is average normal. The U/B values thus calculated are expressed by the higher part of the curve of Fig. 1.
If the urine volume is below 2 cc. per minute, the formula which holds is that of the standard clearance, Cs = U √ V/B, and the average normal value of Cs is 54.
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