Limited value of the fractional excretion of sodium test in the diagnosis of acute renal failure.

The excreted fraction of filtered sodium (FeNa) was examined in 54 patients with acute intrinsic renal failure in whom renal biopsy was performed to determine the morphology of the acute renal disease. In patients with acute tubulointerstitial nephritis oliguric patients had almost constantly elevated FeNa values, whereas in non-oliguric patients with acute tubulointerstitial nephritis this was observed in less than half. The five patients with acute extracapillary glomerulonephritis had an FeNa greater than 1. FeNa values were low, especially in non-oliguric patients who had mild impairment of renal function. The presence of a high FeNa value correlated significantly with severe morphological tubular changes observed on renal biopsy. We conclude that patients with acute tubulointerstitial nephritis cannot be distinguished from those with acute glomerulonephritis by FeNa, as in both types of acute nephritis the FeNa test shows both low and elevated values.