Renal function in the elderly.

Renal size and volume decrease with age, accompanied by intrarenal vascular changes. The number of glomeruli decreases and the mass of the juxtamedullary nephrons falls. The result is a decrease in the filtration area of the glomerular basement membrane and decreased permeability. The glomerular filtration rate (GFR) is reduced with aging. The GFR is approximated by the endogenous creatinine clearance, which falls in parallel with the inulin clearance (the true measure of GFR) and is always greater because of tubular excretion of creatinine. Analytical methods for serum and urine creatinine overestimate its concentration and suffer, to varying degrees, from interferences, making the normal range method dependent. A further uncertainty arises from the use of a correction to standard surface area. Serum creatinine concentration is an insensitive indicator of renal function in the elderly. Deduction of creatinine clearance from serum creatinine concentration, weight and age using one of many formulae gives only approximate values, usually too high, and is unsuitable for debilitated and seriously ill patients. Tubular function, in general, is decreased in the elderly. The ability of the kidney to concentrate urine maximally after water deprivation decreases with age, as does the ability to excrete a water and salt load, particularly during the night. Nocturnal polyuria is common in the elderly. The aged kidney can maintain acid-base balance under normal conditions, but not when subjected to an acid load.