Renal reserve in the elderly.

The increase in glomerular filtration rate after an amino acid load, the so-called renal reserve, was found to be impaired in the aged rat. Renal diseases progress more rapidly in the elderly. Whether the renal reserve predicts progression of renal disease is controversial, however, and generally little information on renal reserve in elderly subjects is available. We examined renal hemodynamics before and after an amino acid infusion in 15 healthy normotensive subjects of young age (median age, 26 years [23 to 32]) and in 10 of old age (70 years [61 to 82]). Median basal inulin (Cin) and paraaminohippurate (Cpah) steady-state clearances were significantly lower in the elderly (102 and 339 mL/min/1.73 m2) than in the young subjects (122 and 647 mL/min/1.73 m2), but virtually all GFR values of the elderly were still within the normal range. The median percent rise of Cin was +16% in young and +17% elderly subjects (independent of gender). A well-preserved renal reserve was also confirmed by two other studies; one in chronically ill elderly patients treated for various nonnephrological diseases using an amino acid load and the other in healthy elderly volunteers in which renal reserve was tested with a protein (meat) meal. The mean absolute renal reserve in the three cohorts studied was between 16 and 26 mL/min/1.73 m2, the oldest patient studied was 89 years. The results of these studies document that in humans renal functional reserve is preserved at least until age 90 years in women and men.