Urinary Diagnostic Indices in Acute Renal Failure

TLDR

The study prospectively evaluated urinary indices to determine the cause of acute renal failure. The authors used the renal failure index (urine sodium divided by the urine‑to‑plasma creatinine ratio) and fractional excretion of sodium to distinguish prerenal azotemia from acute tubular necrosis. Urine osmolality >500 mosm/kg, sodium <20 meq/L, U/U >8, and U/C >40 predict reversible prerenal azotemia, while osmolality <350 mosm/kg, sodium >40 meq/L, U/U <3, and U/C <20 predict acute tubular necrosis, though many oliguric patients fall outside these criteria.

Abstract

A prospective analysis of the value of urinary diagnostic indices in ascertaining the cause of acute renal failure was undertaken. Our results show that in the setting of acute oliguria a diagnosis of potentially reversible prerenal azotemia is likely with urine osmolality greater than 500 mosm/kg H2O, urine sodium concentration less than 20 meq/litre, urine/plasma urea nitrogen ratio greater than 8, and urine/plasma creatinine ratio greater than 40. Conversely, a urine osmolality less than 350 mosm/kg, urine sodium concentration greater than 40 meq/liter, urine/plasma urea nitrogen ratio less than 3, and urine/plasma creatinine ratio less than 20 suggest acute tubular necrosis. A significant number of oliguric patients will not have urinary indices that fall within these guidelines. In this setting, urine sodium concentration divided by the urine-to-plasma creatinine ratio (the renal failure index) and the fractional excretion of filtered sodium provide a reliable means of differentiating reversible prerenal azotemia from acute tubular necrosis.