The Validity of the Calculation of Standard Urea Clearances from Low Urine Volumes.

The "standard urea clearance" is calculated upon the assumption that the urea excretion will vary with the square root of the urine volume, when the latter is below the augmentation limit of about two milliliters per minute. The result of the calculation gives the milliliters of blood which would be cleared were the urine volume one milliliter per minute, if the square root rule were strictly true. M6ller, McIntosh and Van Slyke (1) derived the equation for computing the standard urea clearance as follows: " If C is the observed blood urea clearance (the cubic centimeters of blood, the urea of which is excreted in one minute) with any urine volume output, V, below the augmentation limit, then with the standard urine volume, V8., the corresponding standard clearance, C8, may be calculated by the square root rule as: