IS THE AGED KIDNEY A CHRONICALLY DAMAGED ONE?

Even though the aged kidney has a decreased GFR, it differs in a number of ways from the kidney with chronic renal failure. Then, the healthy person (older than 75 years) and patients with chronic kidney disease (stage 3) share two main physiological characteristics: I) a similar glomerular filtration rate (about 50 mL/min per 1.73 m ), II) a diminished capability for salt and water reabsorption from the renal tubule (2,3). However, despite of these similarities, the aged kidney and the chronically damaged one differ markedly in many other physiological aspects: I) Proximal tubular function is perfectly preserved in the healthy oldest old and their serum erythropoietin and haemoglobin levels are normal. Conversely, anemia secondary to a low serum erythropoietin secretion is one of the main characteristics in CKD patients (4,5,6,7). II) Even though the fractional excretion of urea is increased in both settings, serum urea level is normal in the elderly while it is increased in chronic nephropathy (6,8). III) Serum levels and fractional excretion of magnesium, calcium and phosphorus are normal in the healthy very old population, while CKD patients usually have increased fractional excretion of these substances, in the presence of a normal magnesium, low calcium, and high phosphorus serum levels