URINE 527 



ing this interval pour a little N/2 potassium bichromate solution 1 into each of the 

 two cylinders of the colorimeter (Duboscq's, see Fig. 160, p. 508) and carefully 

 adjust the depth of the solution in one of the cylinders to the 8 mm. mark. A few 

 preliminary colorimetric readings may now be made with the solution in the 

 other cylinder, in order to insure greater accuracy in the subsequent examination 

 of the solution of unknown strength. Obviously the two solutions of potassium 

 bichromate are identical in color and in their examination no two readings should 

 differ more than 0.1-0.2 mm. from the true value (8 mm.). Four or more read- 

 ings should be made in each case and an average taken of all of them exclusive 

 of the first reading, which is apt to be less accurate than the succeeding readings. 

 In time as one becomes proficient in the technic it is perfectly safe to take the 

 average of the first two readings. 



At the end of the five-minute interval already mentioned, the contents of the 

 500 c.c. flask are diluted to the 500 c.c. mark, the bichromate solution is thor- 

 oughly rinsed out of one of the cylinders and replaced with the solution thus pre- 

 pared and a number of colorimetric readings are immediately made. 



Ordinarily 10 c.c. of urine is used in the determination by this method, but if 

 the content of creatinine is above 15 mg. or below 5 mg. the determination should 

 be repeated with a volume of urine selected according to the content of creatinine. 

 This variation in the volume of urine according to the content of creatinine is quite 

 essential, since the method loses in accuracy when more than 15 mg. or less than 

 5 mg. of creatinine is present in the solution of unknown strength. 



Calculation. By experiment it has been determined that 10 mg. of pure crea- 

 tinine, when brought into solution and diluted to 500 c.c. as explained hi the above 

 method, yields a mixture 8.1 mm. of which possesses the same colorimetric value 

 as 8 mm. of a N/2 solution of potassium bichromate. Bearing this in mind the 

 computation is readily made by means of the following proportion in which y 

 represents the number of millimeters of the solution of unknown strength equiva- 

 lent to the 8 mm. of the potassium bichromate solution : 



y :8.i : : 10 :x (mg. of creatinine hi the quantity of urine used). 



This proportion may be used for the calculation no matter what volume of 

 urine (5, 10, or 15 c.c.) is used in the determination. The 10 represents 10 mg. 

 of creatinine which gives a color equal to 8.1 mm., whether dissolved in 5, 10, or 

 15 c.c. of fluid. 



Calculate the quantity of creatinine in the 24-hour urine specimen. 



Interpretation. The daily excretion of creatinine by an adult of 

 medium weight averages about 1.25 grams. The value is nearly con- 

 stant from day to day for a given individual being influenced by the diet 

 hardly at all unless this contains much preformed creatinine (as in case 

 of a heavy meat diet) . The excretion of creatinine is to a certain extent 

 a measure of muscular efficiency and of the amount of active muscle 

 tissue in the body. Relative to body weight less creatinine is excreted 

 by obese persons. 



Creatinine excretion is decreased in disorders associated with mus- 



1 This solution contains 24.55 grams of potassium bichromate to the liter. A pure 

 creatinine standard is to be preferred, see p. 528. 



