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Renal changes that occur with aging mainly consist of impairment in the ability to concentrate urine and to conserve sodium and water. These physiological changes increase the risk of volume depletion and the prerenal type of acute renal failure (ARF) in elderly people. Bladder outlet obstruction caused by benign prostatic hypertrophy is a common cause of ARF in elderly men. Another frequent cause of ARF in the elderly is drug-induced nephropathy. Nonsteroidal anti-inflammatory drugs (NSAIDs) and antibiotics are most often implicated in the development of ARF in the elderly. However, considering the high usage of these drugs, the incidence of drug-induced nephropathy is relatively small. NSAIDs are more likely to cause ARF in patients with congestive heart failure, chronic renal disease (including diabetic nephropathy) or chronic liver disease than in otherwise healthy individuals. NSAID-induced ARF is often of the prerenal type, but may be caused by acute interstitial nephritis (AIN). The presence of heavy proteinuria or nephrotic syndrome differentiates NSAID-induced AIN from AIN caused by other drugs. Antibiotics, especially semisynthetic penicillins, more commonly give rise to AIN associated with peripheral blood eosinophilia and eosinophiluria than NSAIDs. Ciprofloxacin is increasingly reported to cause AIN. Fever commonly accompanies AIN, especially when induced by antibiotics. Aminoglycosides produce ARF by inducing acute tubular necrosis (ATN), which results from the excessive accumulation of myeloid bodies in the tubules. In all cases of ARF it is essential to obtain a good history, to perform a through physical examination, with particular attention to skin turgor, and to measure blood pressure, pulse rate (supine and upright), urinary electrolyte and creatinine levels. Fractional excretion of sodium and the urine:plasma creatinine ratio are reliable indices that distinguish prerenal ARF from ATN. A prompt response to fluid challenge, with an increase in urine output and urinary sodium excretion, and a rapid decrease in blood urea nitrogen, constitutes strong evidence for prerenal ARF. However, these indices are unreliable when prerenal ARF has progressed to ATN or when ARF has an obstructive pattern to begin with. In all cases of ARF, especially in elderly men, urinary tract obstruction should be suspected unless the history is otherwise clear cut. Ultrasound of the kidneys and bladder is a simple, non-invasive and meaningful test that can be used to rule out obstructive causes of ARF. If obstruction is the cause of ARF, ultrasound will be positive; in contrast, urinary obstruction is very unlikely if ultrasound findings are normal in a patient who has been oliguric or anuric for 48 hours or more. Similarly, acute glomerulonephritis, including rapidly progressive glomerulonephritis, should be suspected when ARF is associated with heavy proteinuria. In such instances, percutaneous renal biopsy is essential to document the diagnosis. It is of utmost importance to establish whether ARF is of prerenal or postrenal type, both of which are potentially fully reversible. In contrast, patients with ATN or rapidly progressive glomerulonephritis may not recover, or may only partially recover, their renal function. Haemodialysis and nutritional support are common measures for patients with severe ATN and a highly catabolic state. Corticosteroids and immunosuppressive therapy should be instituted for rapidly progressive glomerulonephritis, in addition to haemodialysis. haemodiafiltration instead of haemodialysis is recommended for patients who are haemodynamically unstable [i.e., with a persistently low blood pressure (systolic < or = 100 mm Hg)]. Haemodiafiltration has been shown to improve acid-base balance and uraemia better than standard haemodialysis. However, despite dialysis, mortality in patients with ARF associated with ischaemic ATN remains high.