Ultramicro Methods in Biochemistry. Vii. the Determination of Plasma or Serum Urea Nitrogen. 8. the Determination of Serum Uric Acid. Ix. the Determination of Serum Creatinine. Lf-22.

Ultramicro methods for the determination of serum urea nitrogen, uric acid, and creatinine have been evaluated and the results are re corded in reports VII, VIII, and IX, respectively. The method for the determination of urea nitrogen is an ultramicro adaptation of the reaction in which a blue color is developed when the ammonium ion reacts with phenol and hypochlorite. Ammonia is released from urea with urease. The entire reaction is conducted on 5/ilof serum or plasma and the final volume of reaction mixture is 2. 205 ml. The procedure for the determination of uric acid is an ultramicro adaptation of the method which depends on the reduction of phosphotungstic acid in the presence of bicarbonate. The phosphotungstic acid reagent contains lithium sulfate as a stabilizing reagent. The reaction is con ducted on lOOfJil of protein free solution prepared from 20^,1 of serum by precipitation with tungstic acid. Comparative analyses show very good agreement between this method and the macro procedure from which it was derived. The creatinine methodisan adaptation of a standard procedure utiliz ing the reaction with alkaline picrate. The reaction is carried out on 100 p,\ of protein-free solution obtained from 40^,1 of serum by precipitation of the protein with tungstic acid. The results obtained with the method are in good agreement with values obtained using the macro procedure from which the ultramicro analysis was derived. Each analytical procedure is evaluated by determination of its reproducibility, and standard deviation analyses are presented.