Residual renal function: a paradigm shift

Residual renal function (RRF) in patients undergoing dialysis treatments is currently viewed as glomerular filtrate that has escaped tubular reabsorption. RRF has been quantified as a clearance of urea or creatinine, or urea + creatinine. A major paradigm shift has followed the recognition that a substantial number of organic anion retention solutes (possible "uremic toxins") are protein-bound and therefore are not readily filtered. These protein-bound aryl compounds are secreted by renal tubular organic anion transporters (OATs). This has led to the recognition that RRF in dialysis patients probably represents not only unreabsorbed glomerular filtrate but also a contribution of renal tubular transporters that secrete organic anions. Tubular secretion of hippurate, indoxyl sulfate, and p-cresol sulfate, protein-bound organic anions retained in the plasma of end-stage renal disease patients, can be quantified and used to evaluate the integrity of a function dependent on active solute transport. Here we propose a shift away from the exclusive "glomerulocentric" view of RRF as unreabsorbed glomerular filtrate and of the progression of renal disease as progressive glomerular loss. We expand the definition of RRF to include the combined renal and tubule functions remaining after a disease begins to destroy nephrons and proceeds to anuria. We propose renewed application of the first principles of renal physiology, articulated in the last century by Homer Smith, to the understanding and monitoring of RRF and progression of renal injury in patients during the sometimes long course of and at the end stage of chronic renal disease.