[Importance of the urinary index in the differential diagnosis of acute renal insufficiency].

The importance of the urinary indices for the diagnosis of acute renal failure is critically reviewed in the light of the literature and our own experience. To differentiate pre-renal from renal acute failure, urine/plasma creatinine ratio (65%) and fractional sodium excretion (99%) provide the best predictive values. Post-renal acute failure is diagnosed by radiological investigation. A theoretical model demonstrates why fractional sodium excretion has advantages on U/P creatinine ratio. The former parameter is independent of urinary volume and glomerular filtration rate, and thus it specifically indicates whether or not tubular function is altered. The U/P creatinine ratio depends on the urinary volume and on the glomerular filtration rate, and therefore this ratio is less specific. The only limitation on the use of fractional sodium excretion in the diagnosis of pre-renal versus renal acute failure appears in patients with an underlying disease able to modify sodium transport by the renal tubules.