102 METHODS FOR THE ANALYSIS OF BLOOD 



Creatine Plus Creatinine. For determining the total creatinine 

 plus creatine in the blood carry out the preliminary precipitation 

 with picric acid just as in the determination of creatinine above. 

 Take 10 c.c. of this filtrate for the determination. Transfer it to 

 a small Erlenmeyer flask or large test-tube. Cover the flask or 

 test-tube with tin foil, transfer to an autoclave and heat to about 

 120 C. for about twenty minutes. The autoclave should not be 

 opened until the temperature has fallen below 100 C. Cool the 

 solution to room temperature, rinse into a 25 c.c. volumetric flask 

 with saturated picric acid solution. Add 1.25 c.c. of 10 per cent 

 NaOH for the development of the color. 



On account of the variations in the creatine content of normal 

 blood two standard creatinine solutions are used. In working on 

 pathological cases a third standard is desirable. These standards 

 contain 0.5, 1, and 2 mg. of creatinine respectively per 100 c.c. of 

 saturated picric acid solution. To 20 c.c. of each of these solutions 

 in measuring cylinders add 1 c.c. of 10 per cent NaOH and allow 

 to stand for ten minutes. By inspection determine which standard 

 corresponds most exactly in color with the unknown and use this 

 for comparison. The standard is usually set at 10 mm. in the 

 Duboscq colorimeter. 



Calculation. Multiply the reading of the standard by 125 and 

 by 0.5, 1, or 2, according to which standard is used, and divide 

 by the reading of the unknown in millimeters. The result gives 

 the number of milligrams of creatine + creatinine in 100 c.c. of the 

 blood examined. 



AMINO-ACID NITROGEN 



Method of Van Slyke and Meyer l 



Principle. The protein of the blood is removed by precipita- 

 tion with alcohol and the amino-acid nitrogen determined in the 

 filtrate by the nitrous acid method. 



Procedure. Thirty to 50 c.c. of freshly drawn blood are 

 mixed with 9 to 10 volumes of 95 per cent alcohol to precipitate 

 the proteins. The volume of the alcohol-blood mixture must be 

 known, but in case it is not convenient to use a graduated cylinder 

 for the mixture, its volume can be taken as the sum of the volumes 



1 Van Slyke and Meyer: Jour. Biol. Chem., 1912, 12, 399. 



