DECREASED FRACTIONAL EXCRETION OF URATE AS AN INDICATOR OF PRERENAL-AZOTEMIA

Although the fractional excretion of uric acid (FE(UA)) is known to reflect extracellular fluid volume changes, the diagnostic significance of decreased FE(UA) in dehydration has not been previously reported. We studied the possible association between low FE(UA) and acute prerenal azotemia, and its diagnostic value, compared with other traditional indices, in discriminating prerenal azotemia from renal parenchymal causes of acute renal failure. In 65 chronic renal disease patients, 174 FE(UA) measurements were obtained from 24-hour urine collections. FE(UA) levels increased as reciprocal serum creatinine levels decreased. All 8 patients with prerenal azotemia showed significantly decreased FE(UA) values compared with chronic renal disease patients with a comparable degree of serum creatinine elevation, whereas all 7 patients with acute renal failure had FE(UA) values higher than those of chronic renal disease patients with comparable creatinine levels. FE(UA) values in prerenal azotemia were distinctly lower than those in acute renal failure (p < 0.001). Patients with prerenal azotemia showed a lower fractional excretion of sodium, a lower fractional excretion of chloride and renal failure index, and a higher urine-to-plasma creatinine ratio than those with acute renal failure (p < 0.05). However, these traditional indices were not useful in discriminating between the two conditions. The urine-to-plasma urea nitrogen ratio and the ratio of plasma urea nitrogen to creatinine showed no statistical difference between prerenal azotemia and acute renal failure. We conclude that, in acute azotemia, a decreased FE(UA) value may represent a reliable indicator of prerenal azotemia in the differential diagnosis of acute renal failure.