Low fractional excretion of sodium in acute renal failure: Role of timing of the test and ischemia

To evaluate the mechanisms for a low fractional excretion of Na (FENa = 1.0) in acute renal failure (ARF) of a sustained nature, causes were determined independent of FENa in 41 patients without volume depletion, obstruction, vasculitis or glomerulonephritis. The 16 patients (39%) with low FENa had lower incidence of preexisting azotemia, lower peak serum creatinine, but higher incidence of renal ischemia and earlier testing (by 1.7 days). Seven often such patients converted to high FENa on repeat, whereas FENa remained high in 15 of 17 patients with initially high values. The initial FENa was a direct function of time from the onset of ARF. Low FENa in acute but sustained renal failure is therefore best explained by (1) milder insults; (2) earlier determinations, and/or (3) superimposed renal ischemia.