As normal urine contains only traces of proteins, the occurrence of the latter in the secretion is looked on as one of the most important symptoms of renal disorder. It is asserted that the albuminous substances eliminated under abnormal conditions are, for the most part at least, derived from the blood. Thus it would appear that albuminuria represents a condition in which the kidneys become permeable to the plasma proteins that ordinarily are utterly unable to pass the renal barrier. It has been established, further, that during periods of such alleged kidney disability the foreign proteins that are present in the food intake do not pass unchanged into the urine. The older idea that the proteins of egg and muscle ingested during severe albuminuria may serve directly to augment the protein output in the urine has finally been disposed of.
Nevertheless it happens occasionally that proteins other than serum albumin,
THE ORIGIN OF URINARY PROTEIN. JAMA. 1928;90(20):1628. doi:10.1001/jama.1928.02690470034013