Comparison of Gasometric, Colorimetric, and Titrimetric Determinations of Amino Nitrogen in Blood and Urine

Of the titrimetric, calorimetric, and gasometric methods now available for determination of amino acids, none is entirely specific. Consequently it has been impossible to tell what proportion of the “amino acid nitrogen” determined in blood or urine with any one of these procedures really represents the amino acids, and what proportion is due to other substances capable of reacting with the reagents used. However, if it can be shown that methods based on several different reactions yield similar results for preexisting amino acids in blood and urine, and afford accurate measurement of added known amounts, it may be assumed with a reasonable degree of probability that the effect of interfering substances is not important. The desirability for a study of the question presented itself to the writers when one of them (Kirk), using the gasometric nitrous acid method, observed regularly in uremic coma a several fold increase in the amino nitrogen of the blood, whereas the numerous clinical studies appearing in recent years with the Folin (1922, a) calorimetric method had disclosed no such phenomenon. The methods chosen for the present study are the formaldehyde titration of Sorensen (1907), the calorimetric method of Folin (1922, a, b), and the nitrous acid gasometric procedure of the authors (Van Slyke, 1929; Van Slyke and Kirk, 1932), each applied to both blood and urine, and the acetone titration of Linderstrem-Lang (1928) applied to blood as described by Zirm and Benedict (1931). Zirm and Benedict appear to be the only previous authors who have reported comparative results on blood or urine by different methods. They analyzed four blood filtrates by their application of the acetone titration, by the Folin calorimetric method, and by