[Kidney functional reserve. Studies in humans]

The glomerular filtration rate (GF) of each nephron can vary, with increases being caused by stimuli such as nephron reduction and ingestion of proteins. This "functional reserve" (FR), detectable by protein load tests, may be lost in extensive renal destruction where the remaining nephrons are in a permanent state of maximal stimulation. If this hypothesis is true, FR determinations would be of greater value than baseline GFR measurements for evaluating the condition of the remaining parenchyma. A very large number of studies have addressed the FR in normal subjects and have used either diets containing variable amounts of proteins or acute loads given orally (approximately 1 g/kg cooked meat) or intravenously (amino acids). All these studies have evidenced increases in the GFR after the load, regardless of its type, but with major variations across studies and patients. Subjects with a single healthy kidney seem to retain a FR whose magnitude is equal to or smaller than the FR in normal subjects. Conflicting data have been reported in more extensive destruction of the parenchyma, where the FR has been shown to either disappear or persist with no change in percentage. Thus, the practical value of GFR determinations after protein loads has not as yet been established and should be specified using better standardized methods that do not rely on creatinine clearance, at least during oral load tests where serum creatinine levels increases and is no longer stable.