One of the important problems in connection with the study of nephritis is the determination of the ability of the kidneys to remove the metabolism products which normally find their way into the urine. Three different lines of investigation have been pursued in the study of this problem, namely, (a) urine analysis, the direct determination of urinary constituents, nitrogen, urea, chlorids, etc.,(b) blood analysis, the determination of similar products in the blood, particularly the so-called uncoagulable nitrogen and the urea, (c) the determination of the ability of the kidney to eliminate certain foreign substances, such as potassium iodid or dyes, indigo-carmin, rosanilin, phenolsulphonephthalein, etc.
The first of these lines of investigation, urine analysis, has contributed very little, except in relation to the elimination of chlorids and water. To anyone familiar with the uncertainties and variations as well as the difficulties involved in nitrogen equilibrium experiments, this
FROTHINGHAM C, FITZ R, FOLIN O, DENIS W. THE RELATION BETWEEN NON-PROTEIN NITROGEN RETENTION AND PHENOLSULPHONEPHTHALEIN EXCRETION IN EXPERIMENTAL URANIUM NEPHRITIS. Arch Intern Med (Chic). 1913;XII(3):245-258. doi:10.1001/archinte.1913.00070030002001