DETERMINATION OF PREFORMED CREATININE 91 



added; it is, therefore, never advisable to work with more than 3 

 to 5 blood nitrates at a time. 



When the amount of blood nitrate available for the creatinine 

 determination is too small to permit repetition, it is, of course, 

 advantageous or necessary to start with more than one standard. 

 If the high creatinine should be encountered unexpectedly with- 

 out several standards ready, the determination can be saved by 

 diluting the unknown with an appropriate amount of the alkaline 

 picrate solution using for such dilution the picrate solution first 

 diluted with 2 volumes of water so as to preserve equality between 

 the standard and the unknown in relation to the concentration of 

 picric acid and sodium hydroxide. 



Preparation of Standard Creatinine Solution. One standard 

 creatinine solution, suitable for both creatinine and creatine 

 determinations in blood, can be made as follows: Transfer to a 

 liter flask 6 c.c. of the standard creatinine solution used for urine 

 analysis (which contains 6 mg. of creatinine) ; add 10 c.c. of normal 

 hydrochloric acid, dilute to the mark with water, and mix. Trans- 

 fer to a bottle and add 4 or 5 drops of toluene or xylene. Five c.c. 

 of this solution contain 0.03 mg. of creatinine, and this amount 

 plus 15 c.c. of water represents the standard needed for the vast 

 majority of human bloods, for it covers the range of 1 to 2 mg. 

 per 100 c.c. In the case of unusual bloods representing retention 

 of creatinine, take 10 c.c. of the standard plus 10 c.c. of water, 

 which covers the range of 2 to 4 mg. of creatinine per 100 c.c. of 

 blood; or 15 c.c. of the standard plus 5 c.c. of water, by which 4 

 to 6 mg. can be estimated. By taking the full 20 c.c. volume from 

 the standard solution at least 8 mg. can be estimated; but when 

 working with such blood it is well to consider whether it may not 

 be more advantageous to substitute 5 c.c. of blood filtrate plus 

 5 c.c. of water for the usual 10 c.c. of blood filtrate. 



Calculation. The reading of the standard in mm. (usually 20) 

 multiplied by 1.5, 3, 4.5, or 6 (according to how much of the 

 standard solution was taken), and divided by the reading of the 

 unknown, in mm., gives the amount of creatinine, in mg. per 100 

 c.c. of blood. In connection with this calculation it is to be noted 

 that the standard is made up to twice the volume of the unknown, 

 so that each 5 c.c. of the standard creatinine solution, while con- 

 taining 0.03 mg., corresponds to 0.015 mg. in the blood filtrate. 



